The Canadian government of Prime Minister Joe Clark was toppled unexpectedly last night over its belt-tightening budget that included an 18 cents per gallon increase in the gasoline tax.

The minority Progressive Conservative government was defeated after only six months in office when it lost support of the small Social Credit Party during a crucial vote of confidence.

Following the 139-to-133 defeat, Clark said he would call on Governor General Edward Shreyer today, presumably with a request to dissolve Parliament and call new elections early next year.

There was immediate speculation that former Liberal prime minister Pierre Trudeau, 60, who last month retired as head of the party, would be called upon to lead the Liberals in the forthcoming campaign.

A parliamentary Liberal caucus was shceduled for today to determine who should be the party's standard-bearer. Sources reached in Ottawa last night said the Liberals would hold a quick convention within the next 30 days to approve the caucus' selection.

Clark, who defeated Trudeau last May to become the younget prime minister in Canada's history, accused the opposition Liberal and New Democratic parties last night of "systematically obstructing" his efforts to curb government spending, lessen the country's dependence on imported oil and improve federal-provincial relations.

The motion of no-confidence, advanced by the leftist New Democrats, condemned the Clark government "for its outright betrayal of its election promises to lower interest rates, cut taxes and to stimulate the growth of the Canadian economy."

Canadian tradition dictates that a prime minister tenders his resignation when his goverment is defeated on a fiscal vote. The new elections probably would be held in February, the first time in nearly a century that national balloting will be held during the harsh winter months.

The sudden government crisis leaves Canada without a federal government at a time when separatist forces in Quebec are intensifying their campaign to take the French-speaking province out of the Canadian federation. A Quebec refernedum on independence is scheduled for next spring.

Hours before the fall of the government, the Canadian Supreme Court declared unconstitutional parts of Quebec's controverisal language law, which makes French the sole official language in the province.

Although the court confined itself to the relatively narrow issue of language used in the courts and the provincial legislature, its 9-to-0 decision seems to call into question other parts of the Quebec law designed to give the French language primacy in all walke of life and business.

Since the court is dominated by English speakers, the ruling was attacked immediately by Quebec separatists as another example of unwarranted federal intervention in provincial matters.

Quebec Premier Rene Levesque immediately seized on the fact that the court yesterday also struck down a similar law in the Western province of Manitoba, where English was declared the sole official language in 1890. Recalling historic bitterness felt by French Canadians, Levesque said, "It took 90 years to challenge the Manitoba law but only two years to overrule that of Quebec."

The unexcepted government crisis is expected to draw national attention away from the secessionist threat of Quebec over the next few months.

It was rumored for most of the day yesterday that the Clark government would fall, especially after the five-member Social Credit Party announced that it would abstrain from the vote in protest against the austerity budget.

With the previous support of the Social Credit members, Clark's 136 members of Parliament enjoyed a tenuous one-vote margin in the House of Commons.

Moreover, three Conservative members, including Foreign Minister Flora Macdonald, were absent attending the NATO conference in Brussels.

Clark's shaky hold on power was weakened over the past few weeks by a series of rapid setbacks including two by-election losses and a dramatic decline in popularity. The latest Gallup polls conducted before the government submitted its budget found only 28 percent of Canadians supporting the government.

The poll showed Liberals winning approval of nearly 50 percent of the population.

Clark's first budget, introduced Tuesday, injected austerity into the holiday season, producing widespread popular dissatisfaction with its proposls to raise taxes on energy, alcoholic beverages and tobacco.

These developments bolstered the courage of the opposition parites to defeat the government last night and force new national elections.

It is theoretically possible that the governor general may ask Trudeau, who despite his resignation remains the titular Liberal leader, to form a new government. Political observers in Ottawa said last night that they considered that unlikely.

Nevertheless, the prospect of an immedate election may resurrect Trudeau's political career despite his resignation if the Liberals are unable to agree quickly on his successor. The name most often mentioned for leadership is Donald MacDonald, a former Liberal finance minister.

Clark, who because the ifrst Conservative prime minister after 16 years of Liberal rule, won only 36 percent of the popular vote last May. His government, the briefest in Canada's history, survived two earlier no-confidence votes with Social Credit support.

Last night, its fate was selaed when the Social Credit leader, Fabien Roy, said he could not vote for a budget that provided for an increase in gasoline taxes.

Following the vote, Clark said in a televised interview, "The opposition parties have decided to disrupt the nation's business. That was not our choice."

Clark's statement hinted at the thrust of the Conservatives' election campaign. Despite their sagging popularity and the failure to live up to their earlier election pledges to cut taxes, the Conservatives will count on public irritation frequently associated with electoral campaigns in wintertime. p

The political crisis in Ottawa makes it almost inevitable that the question of "national unity," as the longstanding feud between the English and French-speaking parts of Canada is called, would again become the focus of public debate.

Yesterday's historic Supreme Court decisions are likely to be used by Levesque and his Parti Quebecois government as ammunition to persuade Quebec voters to endorse sovereignty for the province.

In its ruling yesterday, the court said that the Quebec provincial legislature did not have the authority to pass laws in an area covered by the federal constitution, the British North America Act of 1876, which provides for the use of both French and English as official languages.

As a result, all Quebec laws passed during the past two years were declared invalid.

Apart from its political impact, the ruling is likely to have little practical effect on Quebec. Since 20 percent of Quebec's 6.2 million population are English speakers, the province has been translating all its laws into English. These translations had no official standings, however.

Quebec is now expected to pass legislation giving official status to these translations.

But the Quebec language law has been very popular in Quebec and the court's ruling may undermine the federalist case in the province.

In the case of Manitoba, however, it is not clear what the authorities would be required to do to comply with the court ruling. The province had not made French translations for more than 80 years despite the existence of a substnatial French speaking community around St. Boniface, a Winnepeg suburb.