"Canadians, as we know ourselves, may be an endangered people in a few centuries," Barbara Amiel, an editor for the racy tabloid Toronto Sun, declared in a recent magazine column.

What Amiel really meant, she made clear in Maclean's, the Canadian news weekly, was that because of the low Canadian birth rate and new tides of immigrants arriving in the country, she feared that white Canadians eventually might become extinct.

"Will it matter if the people calling themselves Canadians a few hundred years down the road are largely of East Indian or African stock?" she queried.

Whites of British or northern European descent still make up more than 90 percent of Canada's population, but the faces of the major cities, especially Toronto, have changed dramatically in just two decades, because of the influx of hundreds of thousands of Italians, Chinese, Portuguese, Ukrainians and Greeks.

There are little communities of Caribbean islanders, Hungarians, Basques, Lebanese and other nationalities scattered throughout the metropolis. City officials said they have counted more than 100 cultural or ethnic groups in this metropolitan area of about 2.5 million.

Civic boosters herald the change, saying it has given spice and a new dynamism to the once dour Scottish Presbyterian town, where as recently as 1961, eight in 10 residents could trace their ancestry to the British Isles. Twenty years later, only one in three could.

"I see it as maybe our greatest lesson to the world," Ontario Premier David Peterson said of the new polyglot Toronto in an interview.

"My son's teacher last year was black," he added cheerfully. "You know, my children don't have a thread of racism in them. They have a Chinese friend. Hanukah is as normal to them as Christmas. I've never heard a racist remark from my kids. I've heard a lot from my generation."

Canadians generally reject the American notion of the melting pot, preferring the concept of the mosaic, with each immigrant group retaining its identity and culture while becoming part of the whole.

Until the 1960s, when immigration policies discriminating against Asians, blacks and southern Europeans were eliminated, the mosaic was mostly of a British and northern European character. Since the reforms of the laws and the arrival of the new immigrants, strong government financial support has gone to aid ethnic newspapers, radio stations, special "heritage language" programs in the public schools and folk festivals. Ethnic enclaves are easily identifiable, as street and business signs are often in both English and Chinese, Greek, or whatever is the mother tongue of the neighborhood.

In the new bureaucratic jargon, the key word is "multiculturalism." There is an Office of the Minister of State for Multiculturalism in Ottawa and a Multiculturalism and Citizenship Division in the Ontario provincial bureaucracy here.

"As 'ooh-la-la' is to Paris, and ciao to Rome, and nyet to Moscow, and 'Hey, you're looking great' to Manhattan, so multiculturalism is to Toronto," British writer Jan Morris said in an article for the Canadian monthly magazine Saturday Night on the occasion of Toronto's 150th anniversary two years ago. "Far more than any of the other great migratory cities, Toronto is all things to all ethnicities . . . . I came to feel that Canadian nationality itself was no more than a minor social perquisite, like a driving license or a spare pair of glasses."

It is an open question whether this "determined centrifugalism," as Morris labeled it, will continue. If adults tend to stick to their own communities, the teen-agers and young adult offspring of the new migrants do not.

Droves of them head downtown, especially every Tuesday night, when movie houses charge $2.50. Afterward they sort themselves out according to personal tastes and life styles rather than by ethnic background. In English-style beer halls, heavy-metal or jazz-rock clubs, reggae emporiums or the gay bars, a rainbow of colors and nationalities can be found.

There is also a quiet but palpable restlessness among the new immigrants to find a place in the power centers of Toronto. Most respondents in a survey by the Toronto Star, for a series of articles entitled "Minority Report," complained about subtle discrimination.

Although Torontonians of British or northern European ancestry slipped into slight minority status in the latest federal census in 1981, they are still very much in control of the banks and the politics of the city. A 1980 study by the Canadian Civil Liberties Association found that 17 of 25 private employment agencies surveyed nationally acknowledged that they would comply with client requests for "whites only" for applicants to fill job vacancies.

The incidents that have tended to galvanize the complex Chinese community here have been those occasions when they sensed that they were the target of television commentators or doctors suggesting quotas on foreigners at medical and pharmacy schools.

Canadian-born blacks complain of being lumped together with newcomers from the Caribbean and allege that they are regarded suspiciously as potential shoplifters in downtown stores.

A survey last fall, concentrating on the attitudes of white Canadians, found that many of them were, like editor Amiel, apprehensive about becoming a minority. At the same time, most of them were aware that Canada's low birth rate -- 1.4 children per family -- was smaller than needed to maintain the ethnic proportions of the existing population. Government demographers are predicting an absolute decline in Canada's overall population by the second decade of the next century if the carefully controlled flow of immigration is not expanded beyond its current levels.

One of Amiel's proposed solutions in a follow-up column was for Canada to provide a home for English-speaking whites she assumed eventually would flee South Africa.

The relatively rare violence that has occurred here since the transformation of the city has tended to be within the ethnic groups rather than confrontation between groups.

The most notable examples concern the East Indians. The crash last summer into the Irish Sea of an Air-India flight that had departed from Toronto is believed by Canadian authorities to have been the work of radical Indian Sikh separatists.

For the most part, relations among the ethnic groups have been civil, although there are exceptions. In the corridors of a downtown mall, young Filipino males coming from a disco regularly square off against groups of Portuguese youths. Security guards and police usually quell the incidents swiftly.

Toronto school authorities were startled when, in the mid-1970s, Canadian-born Chinese students and well-heeled newcomers from Hong Kong fought it out on the schoolyard of the city's rigid Jarvis Collegiate academic high school.

"I guess it's a defense mechanism that you tease your own in order to justify your existence," remarked Hong Kong-born school board member Olivia Chow, who was a student at Jarvis at the time.

Canadian-born Chinese call the Hong Kong newcomers "FOBs" for "fresh off the boat." Susan Lee, 23, who was born in Canada of Chinese emigres but speaks no Chinese, shrugged when recalling how the Hong Kong youths tease her. She said they call her a banana: "yellow on the outside and white on the inside."

Lee, who wears her hair in a spiky haircut, works nights busing tables at a popular teen-age "new wave" hangout, has a white boyfriend and talks forthrightly about her aspirations to become a professional dancer and to assimilate into the larger Canadian society.

As for the young Hong Kong men and women her age coming into the city, Lee said in perplexed tones, "I don't know them. They're a breed apart."