Tens of thousands of passengers were stranded Thursday after a thick blanket of freezing fog forced hundreds of flights to be canceled at Heathrow, Europe's busiest airport.

Heated tents, sleeping mats and catering stalls were set up to accommodate irate travelers, many of whom were affected after British Airways canceled 180 flights, including all domestic and some European services.

"It's bedlam. The whole terminal is so packed you can barely walk," said Nicholas Velez, 23, one of about 500 passengers left stranded Wednesday night while trying to return home to Washington, D.C., for the holidays.

About 350 flights have been canceled since Tuesday, when a thick blanket of freezing fog moved in to the city.

"When we flew in last night, you couldn't see the ground," said Velez, who had been transferring through Heathrow. "I've never seen anything like it before."

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Velez said he saw fights breaking out as people scrambled for places in line and criticized British Airways for its management of the crowds.

Outside Heathrow, visibility reached a low of 377 feet, making runways nearly invisible to approaching aircraft, said Keith Fenwick, a spokesman for Britain's Meteorological office. Visibility lower than 3,280 feet is generally considered disruptive for flights.

The fog was expected to continue through Friday -- one of the busiest travel days before the holidays -- raising the possibility that thousands more passengers could be stranded. Eurostar reported a 15 percent spike in traffic as frustrated airline passengers boarded trains to get to Paris and Brussels.

At Heathrow's Terminal Four, the British Airways terminal, lines snaked out of the door as passengers struggled to reach the ticket counter.

Mark Bullock, managing director of BAA Heathrow, said the airport's capacity had been reduced by about 40 percent.

"The cancellations will need to continue as long as the weather conditions prevail," he told the British Broadcasting Corp. Flights at other London airports were relatively unaffected, with nine cancellations reported at Stansted and none at Gatwick.