Philippine opposition presidential candidate Corazon Aquino, concerned about increasing reports of electoral fraud, is urging the United States to boost pressure on President Ferdinand Marcos to ensure a fair outcome and forestall the danger of postelection violence.

"I am really afraid of the violence that will ensue" if Marcos wins the election through massive fraud, she said, apparently worried that the influence of moderates would be undercut by more radical elements who would then see violence as the only solution.

Five campaign security officers from Marcos' party were killed Friday in an ambush by Communist rebels near the Negros Island city of Silay, 300 miles south of Manila, United Press International reported.

Aquino also said she was receiving support from the younger officers in the nation's armed forces, who "see me as their only hope to become generals," and from the Philippines' influential Roman Catholic Church.

Speaking in an interview with Washington Post Co. Chairman Katharine Graham and journalists from The Washington Post and Newsweek, Aquino said she wanted Washington "to really clearly state now before the election that no way will they accept Marcos if it is clearly shown that fraud and terrorism have been committed in order to reelect Mr. Marcos." She was reflecting widespread concern among the opposition that, as it is put here, she may win in the voting but lose in the counting when Filipinos cast their ballots Feb. 7 in the first genuinely contested presidential election since 1969.

Aquino declined to suggest what kind of actions the United States could take if the election were deemed fraudulent, and she conceded that Washington would have to deal with a reelected Marcos "whether he was legitimately installed or not." Therefore, she said, "what is really most crucial and most important is that you exert all pressure before the election." After the election, she added, "it will be too late."

The United States has been pressing Marcos to keep the election clean, but there generally has not been the impression here among political observers that any real muscle is behind the U.S. demands.

Yesterday, President Reagan held out the prospect of increased U.S. economic and military aid if the coming elections were credible and the new government adopted basic economic and military reforms.

In Washington, there is a feeling among some analysts that one way for the administration to send a stronger message is through an official U.S. government observer team that Sen. Richard Lugar (R-Ind.) has agreed to head. The team is scheduled to leave for Manila Tuesday, but the White House has delayed releasing the names of other members of the team, partly because of reluctance by some members of Congress to serve.

If the political machine of Marcos' New Society Movement achieves victory for the 68-year-old president through fraud, Aquino said, she would "lead daily demonstrations in Manila and in the provinces" aimed at forcing Marcos to step down. She said the demonstrations would seek to "disrupt the daily activities of metro Manila," but she acknowledged that such activities might be difficult to keep peaceful.

Asked whether she thought the United States could have much influence on preventing determined election fraud, Aquino replied, "I guess not." She added that "when we really come down to it, it is really Filipino pressure" that will matter: "Marcos doesn't care whether he wins cleanly or dishonestly. To him, the most important thing is to stay on, and he knows that if he is head of state all other nations will just have to recognize him. I can keep on crying about how I was cheated, and nobody will pay attention to me."

In referring to her support from the Catholic Church, Aquino said: "While the Catholic Church cannot openly tell the Filipino people to vote for Cory Aquino, they have already done so in many ways. If I didn't have the Catholic Church, I might as well call it quits. They're the only ones with an organization to at least compete with Marcos and the KBL [New Society Movement]."

She said that "this is the first time the Catholic Church is going all out" in a political campaign. The church is considered a powerful ally for the devoutly religious Aquino in this predominantly Roman Catholic country.

Marcos has long been at odds with the influential archbishop of Manila, Cardinal Jaime Sin. The president has received support, however, from the Iglesia ni Kristo (Church of Christ), a Christian sect founded in the Philippines, which is led by a Marcos loyalist.

Questioned about her ability, if she were elected, to gain control of the Philippine armed forces, Aquino said she already had support from junior military ranks.

"Younger officers see me as their only hope to become generals," she said. She repeated her plan to "retire all overstaying generals" who have remained in active service beyond mandatory retirement age and said she has had "very good feedback from the military themselves" on this issue.

Aquino said she had met some members of a reformist group within the military, who felt her security was "really inadequate." She said they "promised that if they ever heard of any plan to liquidate me, they would tell me."

She denied Marcos' charges that she was naive about the threat of the Communist New People's Army, saying: "Not all of those who have joined the Communists are out-and-out Communists. Many of them have joined because they are escaping from military brutality or Marcos repression, and they feel they cannot get any justice while Marcos is head of our government.

"So it will really be these people that I am appealing to," she said. "As far as the hard core is concerned, I am not naive enough to think that they will ever change. They represent a very small minority, and Philippine culture will never accept communism."