A prominent Argentine nationalist who has been a key witness in court investigations involving high-ranking military officials was abducted by armed men this morning in what was described here as a potentially explosive political kidnaping.

Guillermo Patricio Kelly, an eccentric fringe politician and high-profile investigator of military-related scandals, was driving near his home in a Buenos Aires suburb when he was halted and abducted by at least 10 men armed with submachine guns and dressed in military uniforms, according to witnesses.

The incident was seen by several passers-by and a police patrol officer who was dissuaded from intervening by a man dressed as an Army colonel, police reported. There was no report of Kelly's whereabouts by this evening.

The abduction appeared to ignite a crisis in Argentina's charged political atmosphere two months before the scheduled election of a new civilian government. The incident follows widespread reports of threats to judges, journalists and politicians involved in investigations of the military.

Gen. Reynaldo Bignone, Argentina's president, who is leading the armed forces' promised withdrawal from power, expressed "indignation" at the abduction, and the ruling junta went into special session tonight to consider the case. The military was blamed for the incident, however, by Kelly's wife and politicians who said the government had been unable to control violent groups seeking to halt investigations of abuses and block the upcoming elections.

"It is inconceivable that the government still does not have the monopoly of force to stop these alarming events," said Carlos Contin, a leader of the centrist Radical Party. "There is an impression of great instability."

Buenos Aires evening newspapers gave banner-headline treatment to the abduction of Kelly, whose gadfly investigations and court testimony against military leaders recently have added a new dimension to a long and sensational public career.

A one-time gun-wielding nationalist militant who was jailed seven times and escaped twice under previous military governments, Kelly claimed to have collected volumes of evidence linking military officers to assassinations, abductions and the illegal Italian Masonic lodge Propaganda Due, known as P2.

The investigations of the military, which have advanced in some cases beyond the expectations even of military opponents, recently have become one of the most volatile factors in Argentina's move toward democracy. Many military leaders have strongly opposed probes of past abuses, and the government has said that it is preparing an amnesty law to block court actions and trials.

At the same time, the cases have provoked a series of death threats to investigators, judges and journalists by unknown groups widely believed to be linked to extreme military factions. Three judges investigating cases involving the military reportedly were threatened last week, and yesterday a radio journalist said his program was forced off the air and he was repeatedly threatened with assassination following a broadcast interview with Kelly last Friday.

Kelly's role in the investigations reached a high point last month, when a federal judge ordered the jailing of former Navy commander and junta member Emilio Massera in a criminal case involving the disappearance of a business associate of Massera.

Because of Argentina's legal system, Kelly was able to file the original court charges against Massera in 1981 as a private citizen, and he was reported to have provided much of the evidence in the case.

Kelly also has pressed a court investigation against Massera and several other ranking military leaders for their alleged involvement in the P2 lodge. Yesterday, he appeared before a judge to present evidence in two other disappearance cases linked to the military.

Featured almost daily by the Argentine media, Kelly said late last week that he believed that he and several judges investigating cases of murder and corruption involving the military could be killed. This morning, shortly before his abduction, he declared in a radio interview that "Argentina is the prisoner of a mafia."