Spanish Defense Minister Alberto Oliart today outlined details of a military conspiracy to block the Oct. 28 elections here, but he reported no new arrests and was strongly criticized by opposition leaders for failing to take stronger action against suspected plotters.

Oliart said at an emergency meeting of Congress that the government had foiled a military coup conspiracy intended to train artillery units on the royal palace, the prime minister's residence and key government departments on Oct. 27, the eve of national elections.

The minister's half-hour statement added little to what had been learned of the plot during the past week.

"The honorable member has repeated to us what we have read already in the newspapers," an angry congressman told Oliart. The jibe reflected a growing conviction in Spanish political circles that more than a week after the government claimed it had scotched the conspiracy, the alleged plot remains shrouded in mystery.

According to the minister's statement, operational units stationed around Madrid were due to force a military takeover in a plan of action outlined in documents seized when three colonels were arrested in connection with the plot 10 days ago. The colonels were charged last week with conspiracy in a rebellion.

The defense minister said the plan was detailed, well advanced and, on paper, could have succeeded because the plotters sought to neutralize officers loyal to the government, substituting them with fellow conspirators. Oliart added, however, that it was still not known who was in command of the coup plot, what civilian backing it would have received and, most crucially, which officers in operational units would have taken part.

Faced by hostile questioning from left-wing congressmen, the minister said it was not possible to carry out purges among officers, withdraw commands from suspected extremists and initiate judicial proceedings without proof of their involvement in the plot. A Communist leader said that "waiting for firm evidence amounts to waiting for the coup to take place."

Criticism of the government's handling of the alleged putsch focused on the manner in which the jailed ringleaders of last year's unsuccessful coup attempt were able to meet with the colonels arrested last week. Opposition politicians accused the government of being lax over prison regulations of sentenced conspirators and charged that the same localized, well-knit group of extremists in the armed forces and in civilian life were able to plot the overthrow of democracy virtually unhindered whether they were in jail or free.

"The whole of Spain will continue to be on parole until all the conspirators are rounded up and imprisoned," said another critic in the emergency debate. According to Felipe Gonzalez, the leader of the Socialist Party and the favorite to win the elections, "the coup backers are able to move like fish in water."

Press criticism is focusing on the fact that only three colonels have been arrested in connection with the plot. The influential El Pais newspaper said in an editorial Saturday that the public was perplexed over the "scant and confusing information" provided by the authorities.

A columnist in the popular Madrid daily Diario 16 earlier had accused the government of covering up details of the plot, which reportedly involved 200 officers serving in elite field units, including the parachute brigade and the country's main armored division, which are stationed outside the capital.

Civilian officials had indicated all week that new arrests were "imminent," but the Defense Ministry says none has taken place.

One suggestion is that the military hierarchy has blocked civilian probes into its ranks. On Tuesday the Supreme Army Council, the 21 senior generals, pledged its support for the king and the constitution and condemned the "irresponsibility" of the alleged conspirators.

The council had met with the civilian defense minister and it had been widely expected that it would approve further arrests. The absence of such detentions was seen by political sources as evidence that the generals wished to prevent a morale-breaking investigation of field commanders known for their right-wing views.

The sources said a thorough probe would have met with strong resistance in an officer corps that is apprehensive about the consequences of a Socialist government.

Another suggestion for the reason behind last week's confusion is that the incumbent centrist government exaggerated the extent of the plotting because it wanted to scare voters away from electing a Socialist government. The underlying fear of many Spaniards is that a Socialist government will act as a spur to potential plotters in the Army who have been forced to lie low again following the arrest of the colonels.