Warren Hu had just settled into his flight from New York to London, where he planned to visit his girlfriend, when he became one of the thousands of passengers whose travel was thrown into disarray early Friday because of a fire near Heathrow Airport.

The plane carrying Mr. Hu, a college student in New York, would be returning to the United States, a crew member announced. “I’m a bit surprised, but we can find a solution, so I’m trying to stay calm,” he said in a text message from the plane.

The fire, at an electrical substation near Heathrow, knocked out power at the airport, one of the world’s busiest, forcing the authorities to close it 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 Vietnam, Brunei and India were scheduled to land Friday morning. Passengers were expecting to board planes taking them to dozens of destinations: Tokyo and Singapore, Minneapolis and Miami.

In the Chinese city of Shenzhen, Lukas Zou was among the travelers waiting for news about their now-delayed flight to London. Mr. Zou, who works at a trading firm, said he learned of the Heathrow shutdown just as he was about to board his Shenzhen Airlines flight, 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. He said that Shenzhen Airlines was arranging hotel rooms for the affected passengers, and that their flight might now be rescheduled for 2 a.m.

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