Spain's Defense Ministry was struck here today by a salvo of antitank grenades that left eight persons injured -- two of them seriously -- in one of the most audacious attacks ever attributed to the ETA, the Basque separatist organization.

The attack, in which grenades were fired by remote control from homemade rocket launchers mounted on the roof rack of a parked car, was the second in Madrid in a week and followed increased cooperation between France and Spain against ETA terrorist activities.

New efforts between the two governments to combat the ETA problem were signaled by the French foreign minister's announcement that Basques living in France and suspected of terrorist activities would be deported to Spain, and by the expedited expulsion over the weekend of an alleged ETA member living in southwest France.

The car used in firing the grenades was parked 200 yards from the building in a cul-de-sac across the broad Paseo de la Castellana boulevard. The car blew up shortly after the attack, injuring several bystanders and damaging nearby vehicles and buildings.

A Defense Ministry statement said six grenades hit the facade of the imposing concrete building's fifth floor, just above the offices used by the defense minister. Several windows were broken and an admiral in the ministry's personnel department was hurt by broken glass. A grenade that fell short of the target hit a bus and injured a passenger.

The attack bore the hallmarks of ETA (Basque Homeland and Liberty), which has used the car grenade launchers in the past, and is believed to be the work of a group of clandestine Madrid-based ETA activists known as the "Spain Commando."

The ETA has admitted responsibility for a July 14 car bomb attack in Madrid that killed 10 policemen.

That attack was linked to the deportation by the French authorities to Togo of Domingo Iturbe, a reputed ETA leader who had been serving a jail sentence in France for violating residency requirements.

Today's blasts appeared linked to the unprecedented transfer by the French security forces to their Spanish counterparts of Jose Varona Lopez, said by Spanish police to have been active in terrorist attacks in Madrid. Varona Lopez was detained by French police in the village of Ciboure, near the border town of Hendaye, on Friday evening and passed on to the Spanish security forces over the weekend.

Varona Lopez was not subject to extradition procedures and did not come before a judge in France prior to his transfer to the Spanish police.

The weekend move was a departure from previous French treatment of Basque refugees and ETA suspects who, as in the case of Iturbe, generally have been expelled to third countries.

French Foreign Minister Jean-Bernard Raimond, however, appeared to indicate a change of tactics by the French government when he said in a weekend radio interview that "there will, perhaps, be further expulsions of Spanish refugees. . . in the coming days." In the interview with Radio Monte Carlo, which was widely reported in today's Spanish press, Raimond said that this was "a normal procedure between two law-abiding states."

About 800 Spanish Basques, among them the hard-line members of ETA, are believed to be living in the border area of southwest France. Until recently, the area was considered to be a safe haven for the separatist organization.

The surprise transfer of Varona Lopez prompted outbreaks of rioting in areas of the Basque country in northern Spain over the weekend.