Seedy conditions are common on summer days at London's Heathrow Airport, where standby passengers doze on the floor and potato chips are frequently trodden underfoot. But even by Heathrow standards, today was particularly squalid.

Thousands of would-be passengers spent the day at the airport, mired in chaos and uncertainty caused by the American air traffic controllers' strike and its backing in Canada.

"It's going to be a day of disruption of travelers," a Heathrow spokesman said early this morning. It turned out to be an understatement.

Hundreds of weary passengers, some of whom had waited for more than 24 hours, sprawled on the floor across the whole of the main departure area -- napping, playing cards or trying to have impromptu picnics. By midday, hardly anyone even bothered to look at the flight indicator board, which bore news of flights that were "canceled," "delayed" or "to be advised."

Some young travelers were planning to share cheap hotel rooms if their flights were canceled. But the disorganization was such that most people preferred not to leave the terminal, fearing their flights would be unexpectedly announced.

"It's so confusing," said Frank Bovat from Boston, who was trying to get home on TWA. "We arrived at 9 this morning and they said the flight would leave at 1:30. Then they said it was delayed till 2:30. Finally at 4 o'clock they told us it had been canceled."

Bovat, his wife and two daughters were on the last day of their vacation. "We don"t know what to do now," he said. "But we won't leave the airport. We plan to rough it tonight and see what happens tomorrow."

Of the 19 scheduled flights to North America, only seven left from London by early evening, and some of those were delayed for up to 18 hours. British Airways canceled more than half of its flights, suspended passenger check-ins for North America and put up a notice saying, "It is impossible to predict departure times."

Pan American said it had only canceled two flights and that others were "combined." But Pan Am suffered delays of more than 24 hours.

Three of Trans World Airlines' six trans-Atlantic flights from London were canceled, and those that left were delayed by an average of seven hours, officials said.

Britain's air traffic controllers will meet on Friday to decide whether to support their American colleagues.

"The growing confusion at London airport will be taken into account when they meet," a spokesman for their union said.

The union has already denounced the dismissals and arrests of striking American controllers as McCarthy-like tactics.

"I slept here on the floor last night," said Daniel Kress from Montana, who was booked with Pan Am on yesterday's flight to Seattle. "There was no problem. We checked in and got on the plane. And after sitting there for four hours they told us the flight had been canceled."

Kress, who is a student, said he was running short of cash and did not know what he would do if he could not get back home.

Although there were no outbursts of anger, the strain among passengers far from home was obvious.

Brian Rasmussen, a Pan Am employe on standby, said that "everybody's still good-humored because they hope to get out tonight. If they don't, you're going to get a lot of anger and frustration."

Mothers in rumpled clothes, clutching children, tried to pry information out of harassed airline employes, who would only say that "an announcement is expected shortly."

Pan Am executive Hank Auerbach explained that "we cannot inform them until we know what is happening and we do not want to cancel until it is forced on us."

Some passengers abandoned their original travel plans and were desperately improvising ways to get home.

Luc Peron from Washington, D.C., who has been trying, and failing, to get to New York on British Airways -- he has to go to work on Friday -- was aiming for Canada.

"I plan to take Air Canada to Montreal and drive down," he said. "It could be worse," he added, surrounded by three attractive women he had met at the airport cafeteria. "I've only got $10 left but I don't need food. I've made a lot of friends."

Laina Morris, aged 20 and homesick for Los Angeles, was willing to try for anywhere on the West Coast. "I'm down to my last few dollars," she said. "But someone bought me a hamburger and I'm saving it for tomorrow's breakfast -- just in case."

Student Molly Skousen, who had waited 24 hours for Pan Am's flight to Seattle, said she had only one pound ($1.77) in her pocketbook. "I just want to get home," she said. "I only hope they make the announcement soon."