Political opponents of President Ferdinand Marcos today demanded his resignation and threatened to seek parliamentary impeachment proceedings against him.

The call came after a majority report by an official fact-finding board implicated a close associate and longtime confidant of Marcos, armed forces chief of staff Gen. Fabian Ver, in a wide-ranging military conspiracy to assassinate opposition leader Benigno S. Aquino Jr. last year.

In response to the report, Marcos in effect temporarily relieved Ver and another top general of their commands and ordered their prosecution along with 23 other military men and one civilian named by the board's majority as "indictable" for premeditated murder.

A separate dissenting report issued yesterday by the chairman of the five-member board did not name Ver -- only seven military men, including a third general -- in a limited "criminal plot" to assassinate the Marcos opponent. The seven were also named in the majority report.

Aquino was slain Aug. 21, 1983, seconds after he arrived at Manila airport after three years of self-imposed exile in the United States.

Marcos sent both reports to the Justice Ministry for referral to an ombudsman and trial before a special civilian court that usually hears corruption cases against government officials.

All three generals named in the reports protested their innocence and demanded speedy trials to clear their names.

The majority report, presented to Marcos this morning and subsequently issued to the public, said that 26 persons were found "indictable for the premeditated killing of Sen. Benigno S. Aquino Jr. and Rolando Galman." It said Galman, a small time hoodlum gunned down immediately after Aquino was shot, was set up to take the blame for the assassination as part of the military conspiracy.

Besides Ver, the report named Maj. Gen. Prospero Olivas, the chief of the Philippine Constabulary in Metro Manila; Brig. Gen. Luther Custodio, the former commander of the Aviation Security Command assigned to protect the country's airports; 20 officers and men under Custodio's command; Col. Vicente Tigas, a press relations officer at the Malacanang presidential palace; Col. Arturo Custodio (no relation to the general), a friend of Galman who fetched him at his home four days before the assassination; and Hermilio Gosuico, a businessman who accompanied Col. Custodio.

To replace Ver during his "temporary leave of absence," Marcos appointed the deputy chief of staff and Philippine Constabulary Commander, Gen. Fidel Ramos, 56, as "acting chief of staff." Both Ramos and Ver are distant cousins of Marcos, and both hail from his home region of Ilocos in the northern Philippines.

Ramos, a West Point graduate who served with Philippine forces in Korea and Vietnam, is widely respected among Philippine officers and foreign military attaches, according to military sources. They see in his appointment a chance to improve the image of the increasingly beleaguered Philippine armed forces and remove the taint of the Aquino assassination.

The assassination of Aquino, 50, as he was leaving a China Airlines plane at the Manila International Airport under military escort, plunged the Philippines into a political and economic crisis at a time of mounting pressure on the government by Communist insurgents. The military and the Marcos government blamed the Communists for Aquino's murder, but both reports of the fact-finding board rejected that contention after nearly a year of investigation.

In Washington, the State Department welcomed as "a positive development" Marcos' actions in sending both majority and minority reports to a special court for prosecution, Washington Post staff writer Don Oberdorfer reported. The department, in an unusually strong public signal of its views, had issued a statement Tuesday suggesting that Ver be prosecuted if commission charges were made against him.

Spokesman John Hughes, asked whether he is confident that court proceedings would be honest, said that the United States "will maintain a healthy interest in the progress of the case" against those accused of murdering Aquino. Hughes praised the board for carrying out "an impressively thorough investigation" and said this testifies to "the vigor of democratic traditions" and respect for law in the Philippines.

Both reports insisted the killer was one of Aquino's military escorts, but neither report mentioned a motive for the murder or made any recommendation about specific charges to be filed against those named in the conspiracy. The legal status of those named in the two reports is not immediately clear. Western diplomats said the case has the potential to be bogged down in the legal proceedings.

The majority report, with its wider scope, was greeted with wild applause and chants of "Marcos resign" when it was presented to the press and public today at the board's auditorium. By contrast, the author of the dissenting report, board chairman Corazon Agrava, 69, a retired appeals court justice, was roundly booed at the ceremony and, close to tears, abruptly adjourned it before the majority reports could be read.

However, the other four board members remained in the auditorium after Agrava had left and had their conclusions read aloud to the public.

Shortly afterward, two of the board members, businessman Dante Santos and corporate lawyer Luciano Salazar, tried to leave the country with their families but were stopped at the Manila airport by immigration officials. The officials cited a request from a lawyer representing the military during the board's hearings to prevent all witnesses and members of the board, its legal panel and administrative staff from leaving the country after the report was issued in order to proceed with the subsequent judicial processes.

However, President Marcos later ordered authorities not to prevent the departures of Santos and Salazar. He called the affair a "mix-up" and said the government "does not intend to harass the Agrava board officials."

While the authors of the majority report received wide praise from the Philippine public, members of the Aquino family and Marcos opponents criticized the report on grounds it did not go far enough.

A statement issued today by six opposition legislators on behalf of an opposition coalition in the Philippine Parliament accused President Marcos of complicity in the assassination and demanded his "immediate resignation."

As commander-in-chief of the armed forces, the statement said, Marcos "is morally, legally and politically responsible" for Aquino's murder.

"It is very improbable, in fact inconceivable, that a conspiracy of such magnitude would have been accomplished without the knowledge or consent of President Marcos," the statement said.

"The majority report of the board officially confirmed what the Filipino people had known all along, that the assassination of Sen. Aquino was the result of a military conspiracy," the statement said. "Unfortunately, the report falls short of the people's expectations. The board did not go high enough nor probe deep enough."

The opposition legislators also demanded that those named in the majority report be detained without bail and prohibited from leaving the country until criminal cases against them are concluded.

A separate statement issued by the United Nationalist Democratic Organization, headed by former Sen. Salvador Laurel, backed the call for Marcos' resignation, but added that "We are under no illusion that he will resign."

Therefore, it asked all members of Parliament "to respond to the clamor of the people by commencing immediately impeachment proceedings" against Marcos on grounds that bear responsibility for the Aquino assassination. The opposition does not have a majority in Parliament.

Laurel's statement said the four board members who wrote the majority opinion -- Santos, Salazar, labor leader Ernesto Herrera and university vice-president Amado Dizon -- "deserve full credit for their rectitude and courage. But the majority report does not name the prime mover of the military conspiracy. Like Justice Agrava, the four members also failed to identify the person who gave the order to kill Sen. Aquino."

Further criticism came from Aquino's widow, Corazon, who complained that the report "does not answer the question which has troubled me since Aug. 21, 1983: why was Ninoy Aquino's nickname assassinated by the military?"

She expressed disbelief that the assassination "was planned and executed without Mr. Marcos' foreknowledge or expressed approval." Asked if she believed Marcos had ordered the murder, she replied, "Yes, I do," but said she could not prove it.

For his part, Marcos today stressed that he was giving the more damaging majority report the same treatment he gave the dissenting report presented to him yesterday by Agrava.

"The findings against all those named by the two reports will now form the basis for a judicial prosecution under the jurisdiction of the civil courts, instead of a military court martial," Marcos said. He emphasized his government's "desire and determination to push the case through to a final resolution."

Marcos called the findings of both reports "most serious and grave," but said, "they are also shadowed by serious controversy and differences of opinion."

To expedite the case through the courts, Marcos said he had granted Ver and Olivas "leaves of absence" to prepare their defenses.

Custodio, who was named in both reports, yesterday was relieved of military duties and confined to quarters on Marcos' orders.

In a letter to Marcos today, Ver said, "I proclaim my innocence to the whole world.

"I never imagined in my wildest dreams that I would be implicated in the Aquino case," Ver said. The findings, he said, were "like a bolt of lightning from a clear sky" that "struck me down without any chance of defense."

He called the majority report's conclusions "far-fetched" and "no better than a series of inductive arguments, most of which are not based on facts. . . ."

Ver declared that "the wounds now inflicted upon me come from an unknown enemy which has succeeded in what I consider the conversion of the board into a tool to destroy the protectors of the republic."

But "as a good soldier," he said, he would face his current ordeal and fight "the enemies that are now trying to destroy me."