Army officers were plotting to oust Ferdinand Marcos when the former president discovered their plan, officers who were privy to the plot disclosed today. By ordering their arrests, Marcos triggered the mutiny that brought him down this week.

One of the Philippine officers involved said the reformist officers, who offered confirmation for Marcos' widely disbelieved assertions of a military plot against him, received important intelligence information through the U.S. Embassy, American military authorities and military attaches from neighboring countries once the mutiny began.

Information was provided on the disposition and activities of forces loyal to Marcos, including some intelligence gleaned from U.S. interception of communications, he said.

In addition, Philippine Air Force planes, ordered by Marcos to attack a rebel base on the eastern outskirts of Manila, were allowed to land at a U.S. air base 50 miles north of the capital when the pilots decided to defect to the rebels and purposely ran low on fuel, the source said.

A U.S. Embassy spokesman, Allan Croghan, confirmed that Philippine Air Force planes had landed at Clark Air Base Sunday and Monday and had not taken off again on operations. He had no comment when asked whether U.S. interception of the Marcos forces' communications and other military information had been passed on to the reformists.

The revolt against Marcos by reformist troops succeeded in toppling him Tuesday after massive crowds turned out to back the rebel cause, and military units loyal to Marcos defected to the rebels or refused to fight them. The outcome -- a largely peaceful, even fiesta-like "revolution" that forced Marcos to flee and installed opposition leader Corazon Aquino as president -- was hailed by Secretary of State George P. Shultz as "one of the most stirring and courageous examples of the democratic process in modern history."

The military mutiny was led by Marcos' defense minister, Juan Ponce Enrile, and his armed forces vice chief of staff, Lt. Gen. Fidel Ramos.

But it was not immediately clear whether Enrile or Ramos had been involved in the secret plotting that one military source said had begun "three or four months ago." The reports that surfaced here today implied that Enrile had been made aware that reformist officers in the military had come to the conclusion that Marcos should be removed from office if necessary but that he had not been part of the plotting.

In an interview today, a reformist leader, Navy Capt. Rex Robles, confirmed that reformist officers had drawn up an overall plan that would lead to Marcos' ouster.

"We put together various options, and [Marcos] stumbled onto one of the options," Robles said at the Defense Ministry building at Camp Aguinaldo. He said the plans included an "extreme option" of storming the palace, taking Marcos prisoner and forcing him to resign. But he insisted that no decision had been made to implement this plan when Marcos announced that he had discovered the plot.

It now appears that the discovery of the military plotting did cause Marcos to issue orders to arrest members of the reformist officers' movement and Enrile, which Enrile and some of the officers learned of separately early last Saturday.

Enrile and Ramos barricaded themselves in Camp Aguinaldo, site of the Defense Ministry and the Armed Forces General Headquarters, and called on Marcos to step down as president. They announced that they were withdrawing support from Marcos and backing Aquino, who they said had been cheated of victory in a Feb. 7 election. Enrile said he had just evaded arrest and was prepared to fight to the death if Marcos launched an attack, as expected, on the highly vulnerable camp.

Several hours after the mutiny began Saturday afternoon, Marcos held a press conference in which he charged that the action by Enrile and Ramos was triggered by the security forces' discovery at his Malacanang Palace of a plot to stage a coup d'etat and assassinate Marcos and his wife, Imelda, that night. On live television, Marcos presented three officers attached to his Presidential Security Command who confessed involvement in the plot.

At the time, the allegations were widely dismissed because of Marcos' low credibility. His election opponent, Aquino, publicly had called him "an inveterate liar," and a number of Philippine and foreign observers judged the claims to be a concoction to explain the mutiny.

In fact, however, reformist officers now concede that Marcos was making exaggerated claims about a real plot.They said it was not on the verge of being implemented and did not involve any assassinations.

The three officers who confessed Saturday were released after Marcos left the country, and one of them, Maj. Saulito Aromin of the Philippine Army's 49th Scout Ranger battalion, told a French reporter at the presidential palace Wednesday that the reformists had hatched a plan to storm the palace, take Marcos prisoner and force him to resign. He explained that he was a staunch anticommunist and that he felt Marcos was losing the war against Marxist insurgents.

Essentially, Robles said, the plan was to "get into the palace and give him [Marcos] a choice between war and peace. The object was to talk to the president and tell him to step down." But any talk of killing him, Robles said, "was quickly squelched."

A knowledgeable foreign military attache confirmed that reformists had a plan to "snatch Marcos, but not assassinate him."

The main movers behind the plan were members of an elite 150-man unit loyal to Enrile, reformist sources said. Equipped with a variety of exotic weapons and commando training, the group is led by a young Army colonel, Gregorio (Gringo) Honasan, Enrile's security chief, and Air Force Lt. Col. Eduardo (Red) Kapunan, the defense minister's intelligence officer.

The plot, Robles said, involved members of the Presidential Security Command, Marcos' praetorian guard headed by Col. Irwin Ver. He is the son of the former armed forces chief of staff Gen. Fabian Ver, the ousted president's closest and most loyal aide.

The "most popular" option to unseat Marcos, Robles said, was an elaborate scheme to "liberate" military camps in different provinces while Aquino was pushing ahead with a planned postelection campaign of civil disobedience, strikes, demonstrations and boycotts. The takeover of these camps by reformist officers would have culminated in "a sudden move for Camp Crame," the headquarters, adjacent to Camp Aguinaldo, of Gen. Ramos' Philippine Constabulary.

The first word that Marcos was moving to arrest suspected plotters reportedly came when Enrile was attending a breakfast club meeting Saturday and received a phone call from Roberto Ongpin, Marcos' Trade and Industry Minister. Ongpin is said to have told Enrile that 15 of his security men, including five assigned to him by Enrile, had been arrested early that morning by military police.

Robles also was tipped off in a phone call from Col. Galileo Kintanar, a reformist sympathizer from the Intelligence Services of the Armed Forces.

Enrile went home, where he was joined by his security forces who took him by helicopter to Camp Aguinaldo, where they were preparing to make an Alamo-like stand in the Defense Ministry building.

According to a reformist officer, the rebels appealed to the U.S. Embassy for support -- if only verbal -- but got a "noncommittal" response. Later, however, military attaches from unspecified neighboring countries helped by providing information, often against the orders of their ambassadors, and "prodded the U.S. to do something for us," the source said.

He said the mutineers then got "visual and electronic assistance from [U.S.] monitoring teams," informing them of the positions of Marcos troops, tanks and trucks.