He tried out his high-school French, clowned with a Canadian Mountie and seemed to be having the time of his life on his first trip abroad as president of the United States.

And last night not even a couple of hundred chanting, marching demonstrators seemed to dampen Ronald Reagan's excitement as he and his wife Nancy stepped out for the evening. Canadian Prime Minister Pierre Trudeau was thier host at a gala variety show of all-Canadian talent, and later Canada's Governor General Edward Schreyer was host at a state dinner where guests were the cream of Canadian officialdom.

With 2,000 others watching the gala in the National Arts Centre, the Reagans were swept up with the crowd, clapping and keeping time to folk songs and applauding enthusiastically after Anne Murray sang her big number, "You Needed Me."

The Reagans drew a standing ovation as they rose to leave and, with Trudeau at their side, made a slow promenage through the lobby where hundreds of gala-goers invited by Trudeau waited behind velvet ropes to catch a glimpse of them. Secretary of State Alexander Haig Jr., standing nearby and beaming, called the visit "a wonderful start for the Reagan administration" and said it "clearly forbodes the most cordial and constructive relationship" between Canada and the U.S.

In his after-dinner toast later in the evening, the president said "Our strong defense" was "the foundation of peace, freedom and stability" and noted that Canada and the United States continue to draw close together in times of crisis. Quoting from Canadian poet Robert W. Service, whose poems are filled with tales of taming the wild northlands, Reagan said: "The law of the Yukoni road is that only the strong shall thrive, only the fit will survive."

The president was clearly the star of the show thoughout the evening, out one moment in particular elicited laughter even from the first lady and Trudeau. It came as the trio headed for their limousine and the president spotted an oversized member of the Royal Canadian Mounted Police standing guard outside the door. Sizing up the Mountie, Reagan stood ramrod straight beside him and still could only reach the officer's broad, red-tunicked shoulder.The photographers loved it and Mrs. Reagan, glamorous in black velvet with sunbursts of rhinestones, giggled on the sidelines.

Not laughing were the demonstrators who had dogged Reagan's footsteps throughout the day. They protested U.S. policy in El Salvador and waved banners that said such things as "Canadian people will never become cannon fodder for the U.S." and "Drop jelly beans -- not bombs."

Motioned over to the sidelines by American reporters, Haig sized up the day-long meetings between Reagan and Trudeau: "Complete agreement in the sense of what we are there to do and the need to keep foreign intervention out."

Asked if Trudeau had agreed to increase military aid for El Salvador, Haig stammered a bit before answering that he thought this was a question Trudeau should address, then added, "We didn't have any disagreements."

In fact, Haig said there hadn't been disagreements on any subject, but seemed to backtrack a little when he added, "There were some differences we have to work out -- these are ongoing, dynamic, bilateral issues."

Earlier in the day, Nancy Reagan drew little attention as she moved about Ottawa, leaving crowds and controversy for her husband.

She was understated in her brief public appearances at a school for mentally retarded children and at the National Museum of Man.

The president tossed off a couple of words in French on two occasions. "Merci, " he told a welcoming crowd at the airport, and "C'est un plaisir [it's a pleasure] to be here with you today," he told another crowd later at Parliament Hill.

The first lady, however, stuck to English in a toast she made at a ladies' luncheon given by Lilly Schreyer, wife of the governor general. Alluding to Canada's efforts in helping six captive U.S. diplomats flee Iran last year, she told the group of 18 distinguished Canadian women, "When you're in trouble you turn to your friends. We turned to you, and you certainly came to our aid."

Guests at the all-women luncheon included Marion Dewar, mayor of Ottawa; Karen Kain, the prima ballerina of Canada; and female members of Parliament. U.S. Chief of Protocol-designate Leonore Annenberg attended the luncheon with the first lady.

At the Clifford Bowey School for the Mentally Retarded, a minor flap preceded Mrs. Reagan when a member of the school's board of trustees failed in his effort to have her visit canceled. The board voted 12-to-1 against John Smart's demand that the school not be part of what was called "a public relations" exercise in view of President Reagan's cuts in funds for education in the United States.

Yesterday, board chairman Jane Dobell called Smart "politically oriented but very sincere." She said she had received reactions from parents that underscored Smart's opinions.

"There's a great fear that the cutback in your budget in lowered social services will affect us in society's attitudes here," said Dobell. "She described close connections between the two countries in the treatment of mental retardation and confessed to being "mildly embarrassed" over the incident.

If Nancy Reagan was aware of the disagreement, she did not show it as she moved about the school talking to children. She visited the classrooms where they sang, played games performed gymnastics and showed off a life-training program that teaches the older students to cook, sew, make beds and do laundry.

Kelly Carriere, 18, told Mrs. Reagan she had seen her and the president on television. When the first lady smiled a little tentatively, Kelly consoled, "Don't be embarrassed."

There was lots of hugging and touching and caressing between the first lady and the children, and when someone asked Sue Botvin, 20, how she liked it, she replied, "It's like an autograph."

In a class with 5- and 6-years-olds, most of them were Down's syndrome, a cup of jellybeans spilled onto the floor, causing the first lady to burst into laughter. A little boy noted that it was like "Easter time" as he stooped over to pick them up.

"Every day is Easter time for us," the first lady told him.

In the afternoon at the museum she stopped to spin a roulette wheel called Take a Chance on Immigration. When Queen Elizabeth played it during a visit some years ago, it stopped at a point telling her she'd never made it to the New World but got washed overboard instead.Yesterday the wheel told Mrs. Reagan: "You bought a phony ticket in England. You were left behind."

Mrs. Schreyer and Carol Towe, wife of Canada's ambassador to the United States, Peter Towe, wore hats throughout the day, but Nancy Reagan went bareheaded and got a touch-up in her coiffure later in the afternoon from her personnal hairdresser, Julius, who traveled to Canada with her.