A key Argentine witness in criminal cases linked to the military reappeared outside of Buenos Aires early today, 16 hours after he was abducted by armed men, and said he had narrowly escaped death from a "mafia" seeking to silence him.

Guillermo Patricio Kelly, a flamboyant nationalist and self-appointed investigator of political assassinations and corruption, stumbled into a gas station north of the capital after his abduction yesterday provoked widespread protests.

The 61-year-old activist refused to specify how he had gained release or who had seized him, saying he had agreed to reserve that information for a meeting with a local judge this morning. He said, however, that his captors, although dressed in Army uniforms, were not active-duty officers but part of a powerful paramilitary "mafia gang" that "has more power in this country than the armed forces."

The one-day abduction, one of several reportedly carried out by unidentified armed groups in recent weeks, was significant because of Kelly's role as a court witness in murder and corruption cases involving former Navy commander Emilio Massera and other ranking officers.

With Kelly's release, political tension that yesterday forced a special meeting of the ruling military junta appeared to slacken. But politicians and the Argentine media continued to demand governmental explanations and a crackdown on paramilitary groups they said were threatening the country's move toward democracy.

"Below the ground where Argentines walk lurks a monstrous hydra that every now and then extends its tentacles and claims its victims," said the mass-circulation newspaper Clarin. "The disappearance of Mr. Kelly takes us back in time to the disappearances."

Kelly also demanded action against paramilitary groups and said he would continue to release information on their membership and activities. He was reticent, however, about his experience, saying only that he had been driven around Buenos Aires suburbs in several cars and kept in two different houses.

"I come from being 16 hours face down in a car being subjected to a series of little things," he said, showing bruises. Kelly said his captors questioned him about the murder of a labor union bodyguard that Kelly has blamed on a leading union official, and he later implied that the abduction was carried out by right-wing militants linked to unions.