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.

“It’s a crazy situation,” said Roxanna Bagherzadeh, who was missing Persian New Year’s celebrations with her mother and other relatives in London because her flight from New York had been canceled.

Ms. Bagherzadeh, 27, had intended to have dinner with her grandmother who had traveled from France to Britain for the occasion, but instead, she sat on a plane on the tarmac for two hours before disembarking and returning home.

“I had a quick cry, because I wanted to see my mom,” Ms. Bagherzadeh said.

Passengers at London’s second-largest airport, 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.”

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