BUENOS AIRES, Argentina (AP) _ In one of the boldest terror attacks since democracy was restored more than a year ago, 10 armed men bombed the transmission station of a state-operated radio station, causing extensive damage.

The attack interrupted broadcasting by Radio Belgrano for 81/2 hours Monday, authorities said.

There was no claim of responsibility, but conservatives have criticized the station for its leftist programming.

Buenos Aires provincial police Chief Walter Stefanini blamed the bombing on ''an organization of the extreme right.'' He told reporters the attackers acted ''as a commando-type task force.''

Police said the attack occurred at the transmitter in Hurlingham, a suburb 12 miles northwest of downtown Buenos Aires.

The bombers overwhelmed two security guards and a technician, binding their hands and leading them to a spot about 50 yards from the building, while several packages of explosives were detonated, police said.

The guards and the technician reported that three of the terrorists wore police uniforms and the rest wore nylon stockings as masks.

Before fleeing, the bombers scattered leaflets declaring: ''Now more than ever - democracy or Marxism.''

Short-wave and medium-wave transmitters were destroyed by the blast, and other equipment and wiring was badly damaged, station officials said. Broadcasting was resumed at 11 a.m. using a spare transmittor lent by Radio Excelsior, also state-owned.

Radio Belgrano and Radio Excelsior are the two once-private stations remaining in government hands. All of the country's private stations were nationalized in the 1950s by President Juan Peron's government. Over the years most have returned to private ownership.

The center-left government of President Raul Alfonsin, whose December 1983 inauguration ended nearly eight years of rightist military rule, handed responsibility for Radio Belgrano's programming to a group of liberal and leftist intellectuals and journalists.

The station has been dubbed ''Radio Sandino'' by right-wing political leaders for its staunch support of Nicaragua's leftist Sandinista government and its attacks on U.S. policy in Central America.

The station's director, Daniel Divinsky, said he was concerned that ''the action of 10 persons can leave hundreds of thousands without the voices that feed their desires for communication.''

It was the second terrorist bombing within a week. On Friday, a bomb exploded at the downtown headquarters of the center-right Movement for Integration and Development. Its leader, former president Arturo Frondizi, recently accused the Alfonsin government of leading the country into social and economic chaos.

The Radio Belgrano attack was reminiscent of a raid last Oct. 8 on a federal judge's office in the interior city of Rosario. In that attack five armed men, including at least three in police uniform, carried off files of evidence on alleged human rights abuses by local authorities during military rule.

Nine ex-military leaders of Argentina, including three former presidents, are currently on trial in Buenos Aires, accused of overseeing the abduction, torture, and killing of thousands of citizens who disasppeared in the 1970s.

In the federal court trial Monday, the former editor of an Argentine newspaper testified that the Vatican suspected the military was responsible for the 1976 murders of three priests and two seminarians.

Robert Cox, a native of Britain, was editor of the English-language Buenos Aires Herald from 1969 to 1979, when he left because of death threats for his denunciations of human rights violations by the military dictatorship. He is now assistant editor of the News and Courier in Charleston, S.C.

Cox said the Rev. Pio Laghi, then the papal nuncio in Argentina, talked with him after the December 1976 murders of three Roman Catholic priests and two seminarians of the Palotine religious order. They were killed by gunmen in the living quarters of a Buenos Aires church.

''The nuncio told me that he had the same impression as me, that security forces had been responsible,'' Cox testified. Laghi now is the papal nuncio in Washington.

Cox also said he had learned that the priests' killers had written on the walls with the victims' blood: ''These are people that poison the minds of the young.''