Some travelers were headed to a wedding, some to a funeral. Others were leaving for a vacation in Iceland, taking a business trip to Britain or returning home from a honeymoon in Italy.

But on Friday, tens of thousands of passengers in Europe and beyond were stranded hundreds of miles from their destinations as a power outage at London Heathrow, the main airport serving the British capital, caused major disruptions to air travel.

“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.

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