"Canada's committed suicide," said a despondent supporter of Prime Minister Pierre Elliott Trudeau when the pattern of election returns became clear last night.

The dejected liberal was at a party given by the Canadian Embassy for 600 election night watchers, of whom 400 had actually been invited.

"Canada's in trouble," the man continued. "It's going down the tubes."

Not all Canadians shared his remorse, however, as the vote transmitted by closed-circuit television indicated. "Yippee!" shouted another guest. "It's great stuff. It proves the tremendous advantages of the parliamentary system."

Guests jammed into the N Street NW headquarters of the National Association of Broadcaster, where the Canadian election returns were being fed in by the Canadian Broadcasting Corporation. English was displayed on a large screen in one room, French on a smaller one in another.

"We thought there would be more people who spoke English than French," said an embassy official, explaining the different-sized TV screens. They were right.

The English room had seats like a theater, but the French room had the bar and the food. The buffet, including shrimp, roast beef, ham and small quiches had been prepared for a smaller crowd than actually showed up. When a waiter brought a platter of fried chicken legs, he was engulfed and the contents disappeared almost immediately.

The mood was subdubed and mixed, and by midnight most of the crowd had gone home. A few stayed on to watch final analyses and to close the bar.

When a picture of Progressive Conservative Party candidate Joe Clark came on the screen, one woman in the audience groaned. "The chinless woner," she said. "He ain's glamorous but he's probably going to lead the country."

Real winners in the election were the New Democrats, called a "fringe" party by some analysts. They had nearly 17 percent of the popular vote by midnight.

Victor J. Cieutat, a Canadian now working in Bethesda, said the NDP campaign was "like George Vallace; they weren't really running to get elected, but by taking that percentage of the popular vote they can hope to get a certain amount of leverage. The Conservatives will have to keep them happy if they want to stay in power."

Canadian Ambassador Peter Towe declined to comment on the election results. An aide explained that he was a career civil servant and "completely neutral" in politics. "We will serve whatever government is elected," the aide said.

Embassy personnel were studiously nonpartism but had nontheless provided boxes of campaign buttons of both candidates for anyone wanting souvenirs. The Trudeau buttons disappeared early; the Clark buttons remained readily available for a while.

A running tally of election returns was chalked up on one wall of the larger room by student recorders from the Canadian Center of the Hopkins School for Advanced International Studies. They were not allowed to show their partisan preferences or to drink too much. No such rules inhibited the rest of the crowd which flocked to a large, well-stocked bar.

Outside, there was a small patio notable for fresh air in an atmosphere momentarily free of politics. "I was saying out there. "I really do. At home I drink Canadian Club and ginger ale. People ask me, why waste good Canadian whiskey on ginger ale, but I like it that way." Another guest was talking about the sudden, intense American interest in Canadian polics. "There's a joke about Americans who know more about Margaret Trudeau than they do about Canada," one guest told another.

"I don't suppose that could have helped Pierre," replied his companion.

The existence of four noteworthy parties, as contrasted to two in our country, also complicated matters and gave the election a European flavor. In the color television analysis, the standing of each party was instantly clear because the number of seats in Parliament was presented against a color-keyed background: red for Liberals, blue for the Progressive Conservatives, yellow for the New Democrats and green for the Social Credit Party.

Trudeau's Liberals have been in power since 1963, but as the returns accumulated last night, it began to look as though he would take up the opposition's role. "I wonder whether Trudeau is temperamentally capable of it," one guest mused, watching the returns turn slowly against the Liberals. CAPTION: Picture 1, Pat Thomas with Ambassador and Mrs. Peter Towe and Al Thomas; Picture 2, Canadian nationals, above, watching election returns-Tom Allen, The Washington Post.