President Raul Alfonsin declared a state of siege in Argentina today for a period of 60 days to combat a wave of bomb attacks just nine days before congressional elections are scheduled.

The action, which is sure to provoke controversy here, marked the first time a state of emergency had been declared in Argentina since democracy returned in 1983. Alfonsin's election followed seven years of rule by military juntas.

Officials said the action, which legally authorizes a suspension of civil liberties, would not be enforced against election rallies.

Interior Minister Antonio Troccoli told reporters that the elections, scheduled for Nov. 3, would go ahead as planned. But he defended the sudden clampdown as necessary to curb a recent escalation of violence and to keep in detention without trial those suspected of being responsible for a series of anonymous bombings and telephone threats widely seen as politically motivated.

Government officials suspect that the assaults are the work of right-wing extremists, upset with Alfonsin's decision to place former military junta leaders on trial for human rights offenses and with attempts to curb the influence of the armed forces. These right-wing groups are thought to be trying to make Alfonsin's civilian administration appear weak and to provoke a reassertion of power by the military.

Former president Isabel Peron was the last democratic leader in this South American nation to institute a state of siege. She declared one in November 1974 to quell a rising tide of political violence, and military commanders, who subsequently took control in a 1976 coup, kept emergency powers in force until two days before Alfonsin's election two years ago.

On Tuesday, Alfonsin sought a partial application of emergency powers, invoking constitutional state of siege prerogatives to order the detention of six military officers and six civilians suspected of plotting against the state. But he had stopped short of actually instituting a state of siege to avoid a general restriction of freedom so close to an election.

The 12 men ordered detained all had links with the former juntas, and there were news reports this week of plans to arrest others involved in what officials have called a campaign to destabilize Argentina. The arrest order triggered protests from opposition leaders and legal experts who accused the president of violating the constitution.

On Wednesday, two federal judges affirmed Alfonsin's right to act as he did, rejecting petitions by a number of those arrested questioning the legality of their detentions. But a third judge ruled last night that the arrest decree indeed had been unconstitutional, and one of the 12 was ordered freed.

Further confusing matters -- and undercutting the government's ability to keep its suspects in jail -- another of the alleged plotters received court permission yesterday to leave for Uruguay. Under Argentina's state of siege provisions, a citizen faced with arrest may choose to leave the country.

In Washington, the State Department noted that no curfew had been announced and that the arrest order for the 12 suspects had been reconfirmed, but no other arrests had been ordered.

The Foreign Ministry will ask the United States to extradite one of the 12, cashiered general Guillermo Suarez Mason, believed to be in Miami, news agencies reported.

Troccoli said that today's surprise announcement meant that the arrests of the 12 suspected conspirators could be renewed, and the government issued a separate decree doing so.

Government sources said that a document would be published spelling out the conditions for holding persons under the emergency action. In legal terms, the president's decision empowers authorities to detain people without producing evidence, to conduct searches of homes and offices without warrants and to restrict the size of public gatherings. But Troccoli said that for most people, constitutional rights and guarantees would not be suspended.

The emergency decree said drastic action had become necessary "in the face of new and increasing acts of violence through the indiscriminate placing of explosives and threats and intimidations."

In recent days, bombs have been placed at schools, business establishments and political party headquarters. Lately, military installations and the homes and property of military personnel have become a focus of attack. A blast occurred early yesterday morning outside Troccoli's weekend home in a luxurious northern suburb of Buenos Aires, damaging the minister's carport. Despite the widespread placement of explosives, only one person has been killed.

Today's decree said that calls warning of bombs had prompted the evacuation of 49 primary schools in Buenos Aires yesterday. The Ministry of Justice and Education was evacuated this morning after a bomb threat.