Undercutting a state of siege invoked yesterday by President Raul Alfonsin, an Argentine judge today freed three men detained on suspicion of plotting against the state, arguing that the accused conspirators had been arrested without sufficient proof.

The release order placed the country's judiciary in open confrontation with the executive branch, which had imposed a state of national emergency in large part to enable it to hold the three freed men and nine others on suspicion of conspiring to attack the government. The same federal appeals judge was due to hear petitions from others among the 12 questioning the constitutionality of their detentions.

Top officials hurriedly met to consider options, then announced that the government had appealed today's rulings. The minister of education and justice accused the federal appeals judge who issued the rulings of attempting to assume powers constitutionally granted the president.

"It is alarming to national security that people incriminated by the executive branch for serious public disturbance should be free," the minister, Carlos Alconada Aramburu, said.

Alfonsin yesterday declared a 60-day state of siege, contending that a stronger hand was needed to curb escalating terrorist violence and to respond to earlier contradictory court rulings on the legal status of the 12 alleged conspirators.

Though the government now has assumed for itself the power to suspend civil liberties, the Argentine capital showed little sign today of any departure from normal activities. There was no noticeable increased police presence, and campaign rallies in connection with congressional elections scheduled for Nov. 3 were held throughout the country, confirming the government's promise not to restrict the voting process.

Local papers carried a communique from the Interior Ministry affirming that political parties would be permitted to continue their campaigns. It also said that all rights would be guaranteed and advised Argentines "not to fear any restrictions on their activities."

The statement, together with the absence of enforcement measures, appeared to support a widespread view here that Alfonsin's emergency declaration was meant more as a preelection political gesture than a police action. A spreading series of recent anonymous bomb attacks and telephone threats, thought by the government to be the work of disgruntled right-wing extremists, had put pressure on the two-year-old democratic government to demonstrate it could maintain order.

At the start of the week, Alfonsin responded with the attempted arrest of six military officers and six civilians said to have been conspiring against the state. But instead of seeking the detentions through the courts, the president ordered the arrests directly, saying the constitution's state-of-siege provision gave him the right to do so, though at first he did not declare a state of siege.

After this approach provoked an outcry from opposition politicians and contradictory judgments by the courts on the constitutionality of the action, Alfonsin yesterday imposed a nominal state of siege on the nation and again ordered the arrest of the 12.

Only seven of the 12 suspects were actually taken into custody: retired Army colonel Alejandro Arias Duval, Army Col. Pascual Guerrieri, retired Army major Jorge Granada, Army Capts. Osvaldo Antinori and Leopoldo Cao, journalist Daniel Rodriguez and lawyer Rosendo Fraga.

But five others are still at large: cashiered Army general Guillermo Suarez Mason, who is the alleged ringleader of the purported group, Patricio Camps, Raul Rivanera Carles, Enrique Gilardi Novaro and Jorge Vago.

The Justice Ministry announced yesterday that it had located Suarez Mason in Miami and would ask for his extradition. In hiding since last year from charges of human rights violations, the ex-general served as commander of the country's most important Army corps during part of the seven-year military rule that ended in 1983.

Today's ruling by Judge Luis Enrique Velazco came on habeas corpus pleas brought first by Guerrieri, a colonel assigned to Army headquarters, and then by Rodriguez, a columnist for the conservative daily La Prensa. The two men were freed this morning. Late this afternoon Velazco approved the release of a third, Army Capt. Cao.

The judge asserted that the judiciary branch had the right to review actions taken by the executive and said the courts can demand that a certain amount of "reasonableness" be shown in those actions. To support this argument, he cited the legal textbooks of two constitutional experts who are members of the Senate and belong to Alfonsin's own Radical Party.

The judge said the government had failed to provide sufficient evidence to support the conspiracy allegation and declared unconstitutional at least the three detentions presented to him for review.

Meanwhile, political reaction to the government's announcement of a state of emergency has been mixed and generally split along party lines. Opposition Peronist leaders called the action exaggerated, saying the disturbances did not warrant so drastic a measure. Argentina's main labor union urged the government to call a special session of Congress.