Hundreds of Longest Walk Indians demonstrated in the city again yesterday, this time against the FBI and the Canadian Embassy, while 25 of their leaders met privately with Vice President Mondale and Interior Secretary Cecil Andrus.

About 500 Indians and supporters marched around the heavily guarded FBI headquarters building at 10th Street and Pennsylvania Avenue NW to the beat of tribal drums. Speakers at a rally angrily accused the FBI of abuse and coercion of Indian witnesses in a series of trials growing out of the 1973 siege of Wounded Knee, S.D., and other incidents.

At the Canadian Emabssy, 1746 Massachusetts Ave. NW, a delegation of eight Indians met with Ambassador Peter M. Towe and requested Canadian government pressure on U.S. Justice Department officials in two Canadian-related court cases, while about 150 Indians rallied in nearby Dupont Circle.

While the more militant elements of the Longest Walk coalition, dominated by the American Indian Movement, demonstrated in the street, traditional elders and religious leaders met three hours with Mondale, Andrus and Forrest Gerard, assistant Interior secretary for Indian affairs, in Mondale's office yesterday aftenoon.

Mondale press spokesman Al Eisele said the Indians discussed five basic issues - Indian treaties, human rights, genocide, natural resources and a requested meeting with President Carter. Eisele described the meeting, which was closed to the press, as "forthright and candid." Indians spokesmen at the meeting could not be reached for comment last night.

Mondale praised the Longest Walk as a "dramatic and courageous march to make a case for young people." He said he would examine ways to respond to their pleas, and Andrus said he would convey to Carter their request for a meeting.

The Indians are in Washington to protest pending legislation in Congress that would abrogate all U.S. Indians treaties.

They contend abrogation of the treaties is a prelude to private industrial acquisition of traditional Indian lands and exploitation of badly needed energy resources.

At the FBI rally, speaker after speaker praised U.N. Ambassador Andrew Young's recent statement that there are "hundreds, possibly thousands of political prisoners" in the United States.

Kenneth Tilsen, an attorney who represented Wounded Knee defendants, said the FBI under new director William Webster, despite press reports of reforms, is the same as it was under the late J. Edgar Hoover.

Webster, as a federal judge in the Midwest before coming to the FBI, had a record of anti-Indian rulings. "As a reward, he was given the directorship of the FBI, so we know what to expect from him," Tilsen said.

Richard Mohawk, an AIM member from California, told the cheering crowd: "The people of Cuba and Vietnam rose up and threw off imperialism, and that's what's going to happen here . . . The Palestinians, the PLO, they are indigenous people, they are warriors. We must extend our hand. It is only a matter of time that all indigenous people stand up and shake off the yoke of imperialism."