BOGOTA, Colombia (AP) _ President Virgilio Barco installed eight new Cabinet members Friday in an effort to halt a wave of violence caused by leftist guerrillas, right-wing death squads, drug barons and peasant protesters.

Both his Liberal Party and the opposition Conservatives said changing the Cabinet was not the solution.

''Ministerial patchwork is not going to solve the grave crisis the country is suffering,'' former President Misael Pastrana, the Conservative leader, said in a broadcast interview.

Sen. Alberto Santofimio, a member of the Liberal leadership, told another radio station: ''A change in policies is needed more than a change of people.'' Barco, whose four-year term runs until Aug. 7, 1990, changed eight of his 13 Cabinet ministers Thursday night.

Death squads have massacred more than 100 peasants in four months, drug traffickers have threatened to plunge Colombia into a bloodbath and guerrillas are conducting their longest sustained offensive.

So far this year, guerrillas have blown up the main oil pipeline 41 times, kidnapped 20 mayors, attacked a dozen towns and destroyed bridges, power pylons and communications facilities.

Peasants demanding land and investigation of death-squad activity have held 80 protest marches since Barco became president Aug. 7, 1986.

Demands for presidential action came even from Barco's own Liberal Party after the kidnapping May 29 of Alvaro Gomez, who ran for president twice as the Conservative candidate.

It still is not certain who abducted Gomez or whether he is alive. He is publisher of the Bogota daily El Siglo and the son of former President Laureano Gomez.

The new cabinet ministers are Justice Minister Luis Guillermo Giraldo Hurtado; Agriculture Minister Gabriel Rosas Vega; Labor Minister Juan Martin Caicedo; Health Minister Luis H. Arraut; Mines and Energy Minister Oscar Mejia; Education Minister Manuel Francisco Becerra; Development Minister Carlos Arturo Marulanda Ramirez; and Communications Minister Pedro Martin Leyes Hernandez.

Barco retained the men holding the five most important Cabinet posts: Foreign Minister Julio Londono Paredes, Treasury Minister Luis Fernando Alarcon, Defense Minister Rafael Samudio, Public Works Minister Luis Fernando Jaramillo and Minister of Government Cesar Gaviria Trujillo.

Gaviria is in charge of internal security and was acting president when Barco was in Europe two weeks ago. During that period, 14 people were killed and 40 wounded in a battle between soldiers and peasant protesters.

The protest was one of a half dozen by a total of about 14,000 peasants marching in northern Colombia.

Meanwhile, a group calling itself Christians For Peace held 30 people hostage Friday at the Latin American Bishops' Conference headquarters in Bogota, demanding an investigation of the death squads. The hostages include nine nuns and priests.

About 80 people, including nearly a dozen children, took over the three- story Roman Catholic Church building Wednesday.

Protesters have occupied a dozen embassies, consulates and offices of international relief agencies in the past five years to demand action on grievances that often have little to do with the places occupied. All the sit- ins have ended peacefully.