One hundred Cuban military advisers left here today for home, but the Sandinista defense minister stressed that his government is not lessening its ties to Cuba and that the advisers could return if U.S. "aggression" made that necessary.

In February, Nicaragua said it would send home 100 Cuban military advisers this year as a unilateral step to lessen tensions in Central America. After the House of Representatives voted down a Reagan administration proposal to renew CIA funding to anti-Sandinista rebels last week, it was announced here that the advisers would depart May 2. The move was seen here as part of a gesture promised by Nicaraguan President Daniel Ortega to the Congress if it voted down the administration proposal.

While the Sandinistas say there are only 800 Cuban military advisers in the country, the Reagan administration has insisted that there are at least 3,000.

The Cuban military advisers are said by diplomatic observers here to have guided the organization and training of the Nicaraguan Army, which is thought to number about 50,000, and the country's internal security forces. The Cuban advisers also are said to play a key role in planning counterinsurgency operations against U.S.-backed rebels.

The identities, duties and movements of the Cuban military here are closely guarded by the Sandinistas, but today the top Cuban military official in Nicaragua appeared at farewell ceremonies for his 100 departing compatriots.

Gen. Arnaldo Ochoa, who is reported to have led Cuban counterinsurgency forces in Angola in the 1970s and who has been rumored to have been in and out of Nicaragua since at least 1983, appeared at the ceremony. It was the first time Ochoa had appeared here in public.

Defense Minister Humberto Ortega, speaking before the departing "military specialists," accused the United States of "insolence" in demanding that the Sandinistas "break ties to Cuba and the Soviet Union.

"We are not retreating one step in relations between Nicaragua and Cuba," Ortega said.

Washington Post correspondent Celestine Bohlen reported the following from Moscow:

The Soviet news agency Tass accused President Reagan of venting a "pathological hatred toward the Sandinista revolution" with the announcement of a trade embargo against Nicaragua. Tass accused Washington of "playing up its dispute" with Managua to lay the groundwork for a military invasion.