BUENOS AIRES, JUNE 12 -- A federal judge has declared unconstitutional a new law protecting military officers from prosecution for crimes committed during an antisubversive campaign.

The ruling raises the possibility that the fragile peace President Raul Alfonsin achieved with the armed forces following passage of the law last week could dissolve into a new crisis.

Government officials cautioned, however, that while the judge's decision is an important precedent, it is only the first of what will probably be a series of court opinions on various motions challenging the law's constitutionality. A final judgment from the Supreme Court is expected.

The controversial law protects most members of Argentina's military and police forces from prosecution for crimes committed during the antiguerrilla campaign of the 1970s, on the grounds they were following orders. Senior officers who issued the orders must still stand trial. Alfonsin said this week that he estimates about 50 officers will be prosecuted from among the more than 350 accused.

Today's ruling came in a case involving not military officers but several civilians. The civilians are said to have helped operate a clandestine detention center nicknamed SWAT run out of a hospital headed by an Army doctor.

Earlier this week, the defendants filed a motion asking that the benefits of the new law be extended to them. Relatives of the victims of the SWAT unit also petitioned Judge Manuel Ramos Padilla to declare the law unconstitutional.

The judge ruled that the law constitutes a "privation of justice" and also establishes "an inequality before the law," thus violating two articles of Argentina's constitution.