Canada's two opposition leaders have pounced on remarks by President Reagan as evidence the United States is trying to influence the outcome of Canada's federal election Monday.

Liberal leader John Turner denounced Reagan's brief endorsement of the Canadian-U.S. free-trade deal as unprecedented intervention in Canada's election.New Democratic Party leader Edward Broadbent said Reagan's comments would remind Canadians of how close Conservative leader Brian Mulroney is to Reagan and alert them to the dangers of the free-trade agreement.

Speaking to a U.S. business group in Washington Thursday, Reagan devoted 20 seconds of a 30-minute speech on trade to praising the Canada-U.S. agreement.

"As far as the United States is concerned," he said, "the free-trade agreement is an example of co-operation at its best. It is a testament to the commitment of our two governments to the principles of the open market and to economic co-operation."

After word got out last week that Reagan would mention the Canada-U.S. deal in his speech, anxiety was aroused on both sides of the border that anything he said would be construed as political interference.

Reagan's speech was set several months ago, primarily as a U.S. pep-talk before global trading partners meet in Montreal next month to review the slow progress of multilateral trade talks, known as the Uruguay round of the General Agreement on Tariffs and Trade. Knowing that Canadian opponents and supporters of the deal would examine his remarks carefully, White House officials drafted them with extreme care.