A car bomb, believed to have been detonated by Basque separatists, exploded in central Madrid today, killing five members of the paramilitary Civil Guard and severely wounding three others as they traveled in a security forces vehicle.

Police centered their investigations on the Basque organization ETA (Basque Homeland and Liberty), which has been waging a violent campaign for Basque independence since the late 1960s.

Although the Interior Ministry announced the expulsion of 11 Libyan nationals, among them the commercial attache and two other employes at the Libyan People's Bureau in Madrid, hours after the blast, officials stressed that the decision to expel the Libyans had been made before the bombing and was not connected to it.

Interior Ministry sources said there is no evidence to suggest that ETA is either funded, armed or inspired by Libyan leader Col. Muammar Qaddafi. The sources are wary, also, of suggestions that ETA is seeking to take advantage of present international concern over terrorism or that it is acting in coordination with other urban guerrilla groups.

In Spanish security circles, the Basque separatists in ETA are characterized as a group that obeys its own laws, chooses the timing for its campaigns in accordance with its own interests and is soley concerned with its own Basque-centered objectives. Over the years ETA has been responsible for more than 500 deaths.

Police said more than 40 pounds of explosives had been packed into the car bomb, which was parked close to an intersection along the route normally covered by the Civil Guard vehicle. The lethal charge was detonated by remote control at 7:20 a.m. Madrid time.

The vehicle had been making a routine early morning round relieving and replacing guards at the nearby U.S. and Italian embassies. Its next point of call was the Soviet mission.

Three guardsmen were killed instantly and two more died on reaching a hospital. The force of the explosion opened up a six-foot breach in the wall of a nearby maternity hospital, where no injuries were reported, and windows were blown in within a 200-yard radius.

The hospital's director said it was "miraculous" that none of the more than 30 mothers and babies had been hurt. A wheel from the police vehicle lodged itself inside the delivery room, which happened to be empty at the time of the explosion.

The blast was identical to a car bomb attack in September of last year against a passing Civil Guard vehicle doing a similar embassy run in Madrid. That explosion, which was claimed by ETA, wounded 16 guardsmen and killed a visiting U.S. businessman who was jogging in the vicinity.

Police said they believe the bombers were members of a highly clandestine group of ETA activists who live in Madrid. The group is code-named "Comando Espana," and police say it has been responsible for 18 deaths in the Spanish capital, including those of several generals and other senior ranking military, since the begining of the 1980s.

"Comando Espana," whose members have given police the slip in the past, appeared to have escaped arrest again today. Despite a massive police hunt, which included checkpoints that sealed the city's exits, police were unable to report any progress in catching the bombers.

According to Joaquin Almunia, the chief minister of the Madrid regional government, the activists in the Spanish capital form one of the few remaining groups of ETA gunmen. During the past two years a combination of stepped-up police vigilance and cooperation with the French authorities along the Spanish-French border, where ETA had long maintained its hideouts, has decreased the number of separatist attacks.

In the Basque country of northern Spain, ETA's traditional operating zone, the separatist organization was roundly condemned by the local regional government, which is controlled by the moderate Basque Nationalist Party.

Officials said the expulsions of the Libyans had been decided within the framework of recent talks among senior ministers of the European Community. At a meeting in The Hague yesterday, the European partners agreed on a series of measures to curtail the activities of Libyans suspected of involvement in terrorist activities on the continent.

Spain, which had expelled three Libyan diplomats at the begining of the year, opted to expel another one today in addition to three people's bureau, or embassy, employes who are not technically classified as diplomats. A Foreign Ministry statement said all three had acted in a manner that was incompatible with their diplomatic status.

The Libyan People's Bureau in Madrid had five diplomats and 11 other employes. Officials said the expelled commercial attache's post could be filled by a new appointment but that no new employes would be allowed into Spain to take the place of the two who were asked to leave.

The other eight nationals to be expelled were described as teachers and pupils of a Libyan-funded school in Madrid.