World leaders went out of their way to stress international cooperation Saturday on the eve of the seven-nation economic summit, but a contentious fight over agricultural subsidies threatened to disrupt the hoped-for tranquility.

The 14th annual meeting of the Western world's largest industrial countries - the United States, Japan, West Germany, Britain, France, Italy and Canada - was being held under the tightest security ever seen in Canada's largest city.A 3,000-member security force was deployed around the Metropolitan Toronto Convention Center where the government leaders will begin meeting Sunday, protected by 15-foot-high chain link security fences.

In his weekly radio address, broadcast from the Oval Office, President Reagan looked back to his first economic summit, also in Canada. He said there had been changes for the better in the world economy in the intervening seven years.

"We have led the world toward a remarkable consensus that economic freedom, not state planning and intervention, holds the key to growth and development," Reagan said.

Reagan said the Toronto summit would be a chance to assure that this "great venture to progress continues" through doing more to support economic coordination among the major countries and by removing remaining trade barriers.

This year's summit is expected to be a low-key gathering, in part because President Reagan is a lame duck with only seven months left in office but also because the economy is doing surprisingly well, especially in light of the fears of a global recession that were widespread immediately following the October stock market crash.

Growth in most of the major countries has accelerated this year, driving down unemployment rates to the lowest levels of this decade.

Canadian Prime Minister Brian Mulroney, who is hoping his position as the summit host will give him a political boost at home, said Saturday that it would be wrong to dismiss the summit as a do-nothing affair just because no major initiatives are expected.

"The coordination and cooperation economically that emanates from the summits is a very major contribution towards world growth," Mulroney said during a visit to the rooms where the nearly 3,000 journalists from 50 countries were setting up shop.

"What would we have done last October in the light of the market meltdowns without the Group of Seven," Mulroney asked, referring to the seven summit nations. "The G-7 was able to step in and send signals of confidence to markets around the world."

Reagan, who is attending his eighth and final economic summit, will be the last world leader to arrive on Sunday.

The president said Saturday that efforts to stop illegal drug trafficking and rebuild the economies of Afghanistan and the Philippines also would be on the agenda for the summit as would his proposal, first made last year at the Venice summit, to end farm subsidies by the end of the century.

But Reagan's farm proposal has drawn support from few of the other leaders, with Japan, West Germany and France raising some of the strongest objections.

West German Chancellor Helmut Kohl warned in a speech to the Canadian Parliament earlier in the week that leaders must not move too quickly on the farm question, and Jacques Delors, who represents the European Economic Community at the summit, complained that the U.S. position on farm subsidies threatened to "spoil the atmosphere" of the talks.

In an interview Saturday, Treasury Secretary James A. Baker III acknowledged that the United States was facing intense opposition on the farm question and any breakthrough would be "extraordinarily difficult."