Hundreds of people lined up in the rain at a Georgetown movie theater last night to watch three Canadian films that the Justice Department has decided are "political propaganda" and cannot be shown without a disclaimer declaring that the U.S. government does not approve of them.

Grumbling about federal "intimidation tactics," Justice Department "idiocy" and "blatant efforts to suppress the free flow of information," capacity crowds assembled throughout the evening at the 290-seat Biograph Theatre for four screenings of the documentary films. Two of them are about acid rain; the third, "If You Love This Planet," which has been nominated for an Academy Award, supports a ban on nuclear weapons.

"I find it so infuriating the kind of intimidation tactics the Justice Department is trying to stuff down the throats of the citizens of this country. I would come out here on my last legs," said Ruth Pearl, as she waited to watch the films with her 19-year-old daughter.

Many of those who queued up in the downpour on M Street last night said they would never have heard about or attended the films if the Justice Department, acting under the 1938 Foreign Agents Registration Act, had not ordered that they may not be shown without the disclaimer.

"If you make a ridiculous decision, you change minds. It is so stupid of the Reagan administration, I think it is marvelous," said Ivan Kauffman, who identified himself as a poet who lives on Capitol Hill.

The American Civil Liberties Union, which has announced plans to challenge the decision in court, and several environmental groups took advantage of the government-goes-to-the-movies contretemps last night by holding a prescreening press conference at which the federally prescribed disclaimer was read.

"We do not do this to comply with the directive recently handed down by the U.S. Department of Justice," said Leslie Harris, executive director of the National Capital Area ACLU. Harris read the disclaimer, she said, only as a courtesy to the National Film Board of Canada, which produced the films.

Harris invited the audience to "sing along" as she read the disclaimer, which said, in part, that "dissemination reports on this film are filed with the Department of Justice." The audience at the 5 p.m. screening greeted the reading with giggles.

One environmental leader, James H. Cohen, of the Environmental Task Force, put on a black Lone Ranger mask during the press conference to symbolize what he said was the Reagan administration's "consistent efforts to keep the public in the dark by masking information" on acid rain.

The Canadian government has been pressing the Reagan administration, so far unsuccessfully, to adopt more stringent air pollution standards to curb acid rain.

The screenings last night attracted students who said they had come to see the films because they had heard about them from their teachers.

One teacher, Robert Grey, who directs the Presidential Internship Program in Washington for Phillips Exeter Academy in New Hampshire, showed up with a group of students who had been "assigned" to see the films.

"I think this is an important event," Grey said. "Also, I'm outraged at the idiocy of the Justice Department."