With the top contenders locked in a fierce struggle, Canada's opposition Progessive Conservative Party opened a four-day convention here today that will culminate Saturday in the election of a party leader.

Competition is intense among the seven candidates because, with polls indicating growing disenchantment with the governing Liberal Party, the winner may be in a good position to succeed Prime Minister Pierre Elliott Trudeau. Trudeau is expected to step down within a year, clearing the path for a national election.

Joe Clark, prime minister in 1979 and leader of the Conservatives until January, when he began campaigning, appears to be in the best position, based on samplings of delegates by news media and pollsters.

But Clark, unable to unify the Conservatives during his tenure, is being pressed by two popular party members--Montreal businessman Brian Mulroney and member of Parliament John Crosbie, a former finance minister.

The convention will subject 3,000 Tory delegates from across the country to marathon lobbying by the candidates' supporters. Delegates will have the opportunity to grill the candidates, who will give their make-or-break speeches Friday night.

To win, a candidate must get more than half of the ballots cast. This will probably lead to a series of dramatic votes since none of the candidates appears to have enough backing to win on the first round.

Despite disunity and the lack of a party leader since Clark began campaigning in January, the Conservatives have remained far in front of Trudeau's governing Liberals in voter popularity. The Tories received support from 50 percent of the voters in the latest Gallup poll, compared with 27 percent for the Liberals and 16 percent for the left-leaning New Democrats.

If Trudeau resigns as expected, the Tories appear in a strong position to capitalize on public dissatisfaction with the Liberals, who have been in office for most of the decade.

But much will depend on the Conservatives' ability to field a candidate with broad appeal. The party has been troubled recently by internal dissension.