OTTAWA, SEPT. 24 -- Saying "the ball is very much in America's court," Prime Minister Brian Mulroney insisted today that it is up to the United States to make concessions if it wants a free trade pact with Canada.

Canadian officials said they were waiting for a clear signal from Washington that the Reagan administration is willing to meet their demands for a bilateral tribunal to shelter Canadian industry from U.S. trade laws against subsidies. The tribunal would be empowered to make binding decisions.

The talks fell apart Wednesday when Canada's chief negotiator, Simon Reisman, abruptly walked away from negotiations in Washington and declared an "impasse" over that issue, which is seen here as crucial for a free trade pact.

The Reisman walkout had been planned over the weekend, Canadian officials here said, when Mulroney was told there was little chance of success in the talks.

Politicians, government officials, diplomats and journalists here were unsure whether the walkout was a ploy to gain greater concessions from the U.S. side or a dramatic move by Mulroney to show Canadians that he is willing to abandon the free trade effort, the centerpiece of his economic program, if it comes up short.

Top aides to Mulroney, who had staked his waning popularity on the successful conclusion of a trade agreement with the United States, said the prime minister had no plans to call Reagan in an effort to restart the talks in time to meet a congressionally imposed deadline that is 10 days away.

"You've got to have something to talk about" before Mulroney calls Reagan, said Bruce Phillips, the prime minister's chief communications adviser.

In Washington, President Reagan and his top trade advisers -- Treasury Secretary James A. Baker III and U.S. Trade Representative Clayton K. Yeutter -- decided at a meeting this morning that the president should not call Mulroney with an appeal to restart the talks, administration sources said.

Instead, Baker telephoned Mulroney's chief of staff, Derek Berney, Washington sources said. They added that Baker told Berney that Canada should return to the Washington talks so the two neighboring nations can get back to the business of creating a free trade zone from the Arctic Circle to the Rio Grande.

White House spokesman Marlin Fitzwater underscored the administration message that Canada should return to the negotiations by saying these "complex and difficult" trade issues should be thrashed out at the bargaining table, not through political intervention by heads of government.

But Baker confused the issue slightly by saying in an interview on the "Today" show this morning that, "I think these efforts will involve the very highest political levels of both governments."

Asked by opposition leaders during the House of Commons question period if that meant a summit, Mulroney said a meeting on trade with Reagan would be "inappropriate and perhaps unhelpful" at this time.

Meanwhile, the strong attacks from Canadian officials and opposition leaders on the U.S. position began drawing a reaction from Washington, and the cross fire could make it harder to achieve gains in future negotiations.

Government trade officials here said Canada's "bottom line" positions had been presented to Baker by Finance Minister Michael Wilson and Berney during a secret meeting in Washington Saturday. Officials here said the Canadians were disappointed by Baker's response, and were shocked during the talks that started Monday by what they termed "a regression" in the U.S. bargaining stance.

Administration officials, however, disputed that description of the meeting. One called the one-page list presented Baker by Wilson as "a bunch of generalities."

"The meeting was not a negotiating session and they {Wilson and Berney} did not present a bottom line," another administration official said. "It was a meeting that the Canadians requested to involve the upper political level of both countries."

Other administration officials rejected the Canadian description of why the talks broke down as well as statements that the United States had hardened its negotiating positions in the last two days of the talks.

Fitzwater, in his White House statement, said both sides had made concessions in the 16 months of negotiations, and an administration trade official added, "All they need is to come back to the table and they'll see some more. We're prepared to move on that issue {dispute settlement} if we see from movement from them" on other issues.

But he said the real issue dividing the two sides is not dispute settlement but rather Canada's unwillingness to give up its subsidies.

"The issue is Canada trying to override" U.S. trade laws against subsidies "so they can continue to subsidize with impunity," he said.

In Canada, the issue is seen as the United States' refusing to give up its subsidies while punishing other countries for theirs.

The United States is "consumed with subsidies and protectionism," said Saskatchewan's premier, Grant Devine, on national television this morning.

"They've got a mind-set that everybody's picking on them," he said.

"They want to pick on our subsidies and engage in their unfair trade tactics on the other side of the border," added Joe Ghiz, premier of Prince Edward Island.