Heathrow said it had no choice but to keep the airport closed until 11.59 p.m. on Friday. “We know this will be disappointing for passengers, and we want to reassure that we are working as hard as possible to resolve the situation,” an airport spokesman said in a statement.

If your travel has been affected by the closure, here’s what you need to know.

Should I go to the airport?

Passengers scheduled to travel in and out of Heathrow have been advised not to go to the airport, and to contact their airline for further information.

On Friday, the police turned away passengers trying to reach Heathrow, and cars in the area were diverted.

Most major airlines have updated information on their websites with details about flight disruptions and instructions for rescheduling canceled flights. You can also check their social media channels for updates.

What are my rights?

If your flight departs from an airport in Britain, arrives there on a British or European Union airline, or arrives in an E.U. airport on a British airline, you are covered by British law.

Airlines are legally required to provide care and assistance for delays of more than two hours for short-haul flights, more than three hours for medium-haul flights and more than four hours for long-haul flights. You can check how the law applies to your flight here.

Under the regulations, airlines must provide a reasonable amount of food and drink, a means of communication, accommodations for passengers delayed until the next day, and transportation to and from the accommodations or a passenger’s home.

“The airline must provide you with these items until it is able to fly you to your destination, no matter how long the delay lasts or what has caused it,” the U.K Civil Aviation Authority says on its website.

What if my airline doesn’t give me proper care?

Sometimes during major disruptions like the Heathrow Airport closure, airlines find themselves understaffed, overwhelmed and unable to keep up with passenger demand.

In such cases, the U.K. Civil Aviation Authority advises passengers to pay out of pocket and keep receipts to apply for reimbursement, and to ensure that costs are reasonable. Luxury hotels and alcohol will not be covered by airlines.

More detailed guidelines on how to claim costs or submit a complaint if a reimbursement request is rejected can be found here.

Can I get a refund if my flight is canceled?

Usually, airlines are required to offer a refund or alternative routing if a flight is canceled, but only if the cancellation is the airline’s fault.

“Disruptions caused by things like extreme weather, airport or air traffic control employee strikes or other ‘extraordinary circumstances’ are not eligible for compensation,” according to the U.K Civil Aviation Authority.

Call your airline and see what options it has. Comprehensive travel insurance policies usually cover flight delays and cancellations, so if you have coverage, be sure to read the conditions carefully.

Mr. Miliband said the National Grid, which operates the country’s electrical systems, was trying to route power through a second backup system, but even that was proving difficult given the scale of the fire at the substation, which is near the airport.

The failure of at least one backup system to quickly restore power after such a major outage is likely to be at the center of questions about the reliability of Britain’s infrastructure in the aftermath of the fire and airport closure. Frustrated passengers and some London residents were expressing disbelief and outrage on social media.

Mr. Miliband said the government would focus on understanding the cause of the outage, telling Sky TV: “Obviously, with any incident like this, you will want to understand why it happened and what if any lessons it has for our infrastructure.”

A Heathrow spokesman said the airport had many backup systems designed to support different parts of the operation. The official said that more information would be released later in the day about how the backup systems were affected and why they were not able to keep the airport open after the fire.

National Grid officials did not immediately respond to an email requesting information about the backup systems.

“This will clearly have a significant impact on our operation and our customers,” British Airways said in a statement. The airline was the most affected as it had about 340 flights scheduled to land at Heathrow on Friday, according to data from Flightradar24.

The airline had redirected all flights already on their way to Heathrow on Friday morning to other airports in Britain.

Virgin Atlantic, another British airline, said it had canceled all flights until 9:30 p.m. London time, “with the rest of today’s schedule currently under review,” it said in a statement.

The airline was asking customers to not travel to Heathrow or their scheduled departure airport if Heathrow was their destination and not to contact customer services. Instead, they said travelers should check their flight status on the airline’s website and people with canceled flights would be emailed rebooking details.

Air France had canceled eight return flights from Paris-Charles de Gaulle Airport to Heathrow on Friday, but flights to other airports in Britain were operating as normal, the airline said.

KLM, a Dutch airline, said it had canceled three return flights from Amsterdam to London Heathrow and would rebook passengers. But the airline said that it would try to accommodate passengers with an “urgent need” to fly to London by placing them on flights to an airport near London.

Lufthansa, a German airline, said all customers on flights in and out of Heathrow have been rebooked, but it urged customers to keep their contact details updated.

Nine flights by Qatar Airways between Heathrow and Hamad International Airport near Doha, were affected. For example, one flight was redirected to Frankfurt and another to Brussels.

Air India had canceled most of its flights to Heathrow, though one returned to Mumbai and another was diverted to Frankfurt.

Qantas, an Australian airline, said its Singapore to London and Perth to London services were diverted to Paris on Friday, with buses arranged to take customers to London. Flights departing from Sydney and Perth on Friday and expected to land in London on Saturday morning where still scheduled to run as normal.

On their websites, most airlines say canceled flights are rebooked free of charge. Costs are likely to mount for airlines as they redirect flights and compensate customers for hotel rooms, food, transportation and other costs related to upended schedules.

Shares in airline companies dropped in European trading. Though the losses were easing by midmorning, shares in IAG, the parent company of British Airways, Aer Lingus, Iberia and two other airlines, were down about 1 percent. Air France-KLM shares dropped about 1.6 percent.

Ryanair, an Irish airline, said it had added eight “rescue” flights to its schedule for Friday and Saturday between Dublin and London Stansted airports.

Lauren Leatherby, Melissa Eddy and Liz Alderman contributed to reporting.

First, the planes full of people and cargo already on their way to Heathrow have to find another place to land. Countless passengers have to find hotel rooms wherever they end up. Crews and pilots must take legally mandated rest. Airlines have to figure out how to reroute planes and cargo.

Ian Petchenik, the director of communications at Flightradar24, a flight tracking website, described it as “a million moving pieces that are all trying to get where they need to go.”

Most major international airlines operate flights into Heathrow, which Mr. Petchenik called the “crown jewel of most international airlines,” and it receives flights from the widest variety of air carriers of any airport in the world.

When Heathrow shut down early Friday, 120 flights were already heading there.

Some were able to land nearby, at other London airports or in Birmingham or Manchester. Others were redirected to continental European destinations like Amsterdam and Frankfurt. One British Airlines flight that left from New York’s Kennedy International Airport ended up in Reykjavik, Iceland. A Japan Airlines flight from Tokyo went to Helsinki, Finland.

In Taipei, the day’s only flight to Heathrow took off shortly after 9 a.m. The plane had only made it past Hanoi when it turned around and headed back to Taipei, according to data from Flightradar24.

But most flights ended up neither where they had started nor where they planned to go, setting off a global logistical scramble for hotel rooms and alternate flights as passengers tried to figure out how to make it to sightseeing tours, business meetings and family events.

The high number of rerouted planes makes the intricate work of air traffic control even more complicated than usual.

“You stuff more aircraft into the same airspace, you’ve got more complexity,” said Martin Craigs, who spent decades in the aviation industry, most recently as president of the Aerospace Forum Asia, a Hong Kong-based association that represents aviation equipment suppliers.

Most airports are prepared to handle slightly higher than usual numbers of incoming aircraft, Mr. Craigs said. “The system has been developed to deal with worst-case scenarios.”

Officials in London said they expected the disruption at Heathrow to continue for several days. It could also take days for airlines to rebook the hundreds of thousands of passengers they expect to be affected.

The shutdown of an important global transit hub for such a prolonged period of time is “unprecedented,” Mr. Petchenik said. A rare, similarly extensive disruption occurred in 2010 when a volcano eruption in Iceland grounded more than 100,000 international flights over several days.

“We are stuck,” said James Porritt, who was supposed to leave early Friday for Australia until his flight was canceled. Mr. Porritt said that he and his wife, who were staying at a hotel at Heathrow, were not going to be able to leave until Sunday.

“She is up in the room worrying,” he said of his wife. “She has been crying from stress.”

Passengers at the second-largest airport serving London, Gatwick, which received many flights that had been headed to Heathrow, stood in line at help desks, sat on the ground or frantically refreshed airline websites.

“This place is a mess,” said Patricia Bradley, 69, noting that she had lined up in “about six different places.”

The scene at Gatwick was in stark contrast to the eerie quiet at Heathrow, usually one of the world’s busiest airports. The drop-off zone at Terminal 3 was deserted; runways and airline counters were empty.

Denyse Kumbuka, a teacher who had been trying to get home to Dallas, said that she might have to try to sleep in the terminal because she could not afford a hotel. “I don’t have a choice,” she said.

The power outage, caused by a fire at an electrical substation near Heathrow, forced the authorities to close the airport for all of Friday. They warned that flight disruptions could last for days.

A glance at Heathrow’s flight information board on Friday morning gave a sense of just how big the shock waves from the closure would be. Flights from Brunei, India and Vietnam were scheduled to land, and passengers had been expecting to board planes taking them to dozens of destinations: Miami, Singapore, Tokyo… the list went on.

“I am extremely frustrated,” Tori Allen, 31, who was supposed to leave London for a birthday trip to Iceland on Friday night, said, adding, “It’s all quite chaotic.” She had not heard anything from her airline, she said, but she went ahead and booked new tickets for Saturday morning, for double the price.

Travelers took to social media to complain about the disruption and ask the airlines for help. One wrote that she could no longer run the half-marathon she had prepared for, another that he had had to pull out of a Pokémon Go championship.

In the Chinese city of Shenzhen, Lukas Zou was among the travelers waiting for news about a flight to London. Mr. Zou, who works at a trading firm, said that he had learned of the Heathrow shutdown just as he was about to board his Shenzhen Airlines plane, scheduled to depart at 1:45 p.m. local time.

“With so much luggage, I don’t know if I should laugh or cry. I’ve already booked a hotel room in London, and it can’t be canceled,” Mr. Zou said.

Dozens of airlines fly to Heathrow from about 180 places worldwide. Many planes already in the air were forced to divert elsewhere. Kennedy Airport in New York had the most flights scheduled to arrive at Heathrow on Friday. Some of the more than 20 flights that were supposed to arrive from Kennedy were diverted to Manchester, in northwestern England; to Glasgow; or to Reykjavik, Iceland.

As many as 290,000 passengers traveling in or out of Heathrow could be affected by the closure, according to Cirium, an aviation analytics company. On Friday, 669 flights were scheduled to take off from Heathrow, Cirium said.

Heathrow is the main hub for British Airways, which said that it was redirecting flights to other airports in Britain, where possible.

At Fiumicino Airport in Rome, dozens stood in line at the British Airways counter. The airport’s information board showed that four British Airways flights were canceled on Friday morning.

A group of high school students from Arizona, planning to fly home via Heathrow after a week in Italy, arrived at Fiumicino to disappointment. “We didn’t find out the flight was canceled until half of our party had checked in,” said Angel Brady, a chaperone on the trip. She said that the group would miss its connecting flight to Phoenix.

North American, Asian and Australian airlines were also affected. American Airlines, Delta Air Lines, United Airlines, Air Canada and JetBlue were among those having to divert their Heathrow-bound jets or return them to the airports they had come from.

Nicholas Yong contributed reporting from Singapore, Jonathan Wolfe and Lynsey Chutel from London and Matthew Mpoke Bigg from Rome.

Here is what we know about Heathrow’s closure.

The problem began with a nearby fire.

Heathrow said early Friday that it had been hit by a power outage, caused by a fire at an electrical substation supplying the airport.

The London Fire Brigade said that it had been called to the fire, in western London, around 11:20 p.m. on Thursday, and had deployed dozens of firefighters. About 150 people were evacuated from the area. The fire was under control by around 6:30 a.m., the fire brigade said.

Britain’s energy secretary, Ed Miliband, said it was “too early to say” what caused the fire. He told LBC Radio that so far “there’s no suggestion there is any foul play.”

Counterterrorism specialists are involved in the investigation into what caused the fire, given the location of the substation and the impact on critical national infrastructure, London’s Metropolitan Police said in a statement.

Nearby homes and businesses were also affected by the outage. Some 100,000 homes were without power overnight as a result of the fire, Mr. Miliband said. That number was down to about 4,000 later in the morning.

When will Heathrow reopen?

Heathrow advised passengers not to go to the airport because it would be closed until at least 11:59 p.m. local time on Friday.

The ripple effects are global.

Heathrow’s closure will affect at least 1,351 flights in and out of the airport, according to Flightradar24, a flight tracking website. As the closure was announced early Friday, 120 aircraft were in the air heading to Heathrow.

Some were diverted to alternate airports, including London Gatwick Airport, Amsterdam Airport Schiphol and Charles de Gaulle Airport in Paris, Flightradar24 said. Others turned back to the airports they had left.

It could take several days for airlines to rebook passengers. As many as 290,000 people scheduled to fly to or from Heathrow could be affected by the closure, said Mike Arnot, a spokesman for the aviation analytics firm Cirium.

The disruption caused by the fire was expected to continue into Friday morning, said Pat Goulbourne, assistant commissioner at the fire brigade.

“We expect significant disruption over the coming days and passengers should not travel to the airport under any circumstances until the airport reopens,” a Heathrow spokesperson said in a statement.

Britain’s laws protect many travelers facing disruptions.

Airlines are required to look after you if you are departing from Britain on any airline or arriving there on a European Union or British airline. If your flight is canceled, you should be offered a choice of a refund or alternate travel arrangements at the earliest opportunity, according to Britain’s Civil Aviation Authority.

If you choose to travel at another time, the airline must provide you with a reasonable amount of food or drink, accommodation if your rerouting is not until the next day, and transport to and from the accommodation.

If your flight has been canceled or diverted, check with your airline on its policies.