Opposition leader Corazon Aquino urged supporters at a mammoth rally today to take up a nonviolent campaign of street protests, strikes and economic boycotts against President Ferdinand Marcos, as he sought to defuse postelection tensions by announcing the retirement of controversial armed forces chief of staff Gen. Fabian Ver.

As the Aquino rally was under way, Marcos, in a press conference, warned that the government will arrest and prosecute citizens who engage in civil disobedience and laid most of the blame for electoral fraud on the opposition. He also said that President Reagan had been "wrongly informed" when he issued a statement yesterday questioning the credibility of the election and blaming Marcos' ruling party for most of the fraud and violence that marred it.

On Monday, U.S. presidential envoy Philip C. Habib, who arrived in Manila last night, is scheduled to meet with Marcos and Aquino and is expected to meet with Cardinal Jaime Sin, archbishop of Manila.

Aquino unveiled her program before hundreds of thousands of supporters, many of them waving flags and chanting her name, who gathered in a park on Manila Bay about 12 hours after the Philippine National Assembly proclaimed Marcos the winner of the Feb. 7 presidential election.

Aquino said she would organize "a national day of prayer and nonviolent protest," a virtual general strike in which people would stay away from work and school. It is to be held just after Marcos is inaugurated for a new six-year term as president on Feb. 25. She also asked her followers to boycott certain newspapers, banks and businesses that are run by Marcos associates and to delay payments on their utility bills.

"Organize the areas where you live, factories, offices and schools," Aquino told them. "Show the dictator and the world that we will continue with our protests, that we won't stop until freedom is returned to us."

The rally was one of the largest in recent times in the Philippines, apparently somewhat larger than the final one of her presidential campaign. It demonstrated that despite Aquino's failure to remove Marcos through the electoral process, she retains the ability to mobilize huge numbers of people.

In a news conference at the heavily guarded Malacanang presidential palace, Marcos adopted a combative attitude in response to charges by the opposition and Washington that his reelection was marred by massive fraud, vote-buying and violence that denied Aquino victory.

Referring to Reagan's latest statements about fraud and violence, Marcos said: "I intend to see to it that the correct information reaches him." Marcos called for "serious and fair investigations" of electoral fraud and violence, including offenses by members of his own party.

Marcos said that he interpreted Habib's mission as aimed at resolving "serious doubts as to the information reaching the president and some of the members of the decision-making groups in the United States."

Marcos said, "I'm still the president of the Republic of the Philippines, whatever anybody may say." He added: "I intend to convince everybody that the fraud or violence was committed by the opposition more than by the government."

The 68-year-old president, who has been in power for 20 years, said that "this vote, as far as I am concerned, reflects the will of the Filipino people. We are beating a dead horse by talking about whether the people are supporting me or not. To me, it's over."

He said: "The moment they the opposition and other critics can prove to me that I'm an unwanted -- what they call a dictator -- then I'll get out. I am the president. They are not going to drive me out, because the people are behind me."

[In response to growing pressure from some members of Congress for a suspension of U.S. aid to the Philippines, Marcos, in an interview on the CBS television program "Face the Nation," reiterated that he would seek renegotiation of the two U.S. military bases in the Philippines with or without the U.S. aid.]

Marcos gave no reason for retiring Gen. Ver, who was acquitted last year of involvement in the 1983 assassination of Aquino's husband, former senator Benigno Aquino. U.S. officials had been pressing for the removal of Ver, a close confidant of Marcos, as a means of promoting military reform that is seen as vital in countering a growing Communist insurgency.

However, Marcos indicated that Ver would remain as a "consultant" on a board of generals that is instrumental in formulating military policy. He said Ver, who resigned all his posts, including chief of the National Intelligence Security Agency, would be replaced by Lt. Gen. Fidel Ramos, who would serve as acting chief of staff. Both Ver and Ramos are cousins of Marcos.

Opposition spokesman Rene Saguisag quoted Aquino as saying that Ver's retirement was "long overdue" but that it could turn out to be "all show." Saguisag said the opposition feared that Ver would "remain the real power" in the military. "Ramos is really just part of the furniture of the military establishment" and has "no power," Saguisag said.

As Marcos spoke, the presidential palace was under guard by hundreds of troops in combat gear behind barbed wire. Many of the soldiers carried M16 rifles, while others were armed with truncheons and shields.

Aquino's "People's Victory" rally was meant to repudiate the National Assembly's declaration of Marcos as the winner of the election, which Aquino, church leaders and millions of her followers say Marcos stole from her through systematic fraud and intimidation of voters.

The rally marked a shift to a risky strategy of street activism in her movement's battle against Marcos.

"I ask our fellow countrymen in the government, especially the police and military, not to support a government that is no longer supported by the people," she told the crowd, which gathered in a park on Manila Bay known as the Luneta.

"It is not against the laws of man and God to disobey unjust orders. We hope violence will not come from you. We will move peacefully. If you cannot openly follow us, please don't destroy, disperse, apprehend or shoot us," she said.

A seven-point program of action that she unveiled was based on the more moderate options that had been discussed in preceding days. Though the term "civil disobedience" was heard frequently before today, she did not use it in her speech, and the acts she advocated would not, for the individual, be illegal. She did not, for instance, propose nonpayment of taxes, as had been proposed earlier.

Campaign sources said Aquino was concerned that too radical a program would alienate the Philippine middle class, from which she draws much of her moral and financial support.

The rally recaptured the ebullient mood of her campaign. It began with a series of marches around the city. As they arrived at the rally grounds in early afternoon, they were greeted with cheers, firecrackers and honking horns.

Nuns and priests were scattered among the marchers, evidence of the support that the Catholic church here has given Aquino's new tactics. "We are here with the people," said a student from San Jose Seminary. "We want to remove Marcos."

Priests said mass on the speaker's platform, a flatbed truck. In the crowd before them, children wore the campaign's trademark yellow ribbons and adults waved flags and sang the opposition's anthem, "Bayan Ko" (My Country). Vendors plied the crowd selling peanuts and mangoes.

In her speech, Aquino called for a boycott of seven banks that are owned by the government or Marcos associates. Included was the country's largest, Philippine National Bank. She asked that people withdraw their money and not make new deposits.

The Philippines banking system has suffered a series of failures and takeovers in recent years during an economic crisis. It was not clear what effect Aquino's call would have, especially since many Filipinos have no bank accounts.

In addition, she requested a boycott of progovernment newspapers and a government-run television channel. She asked that no business be done with the San Miguel Corp., the food and beverage producer that is the country's largest corporation and is controlled by a Marcos associate, and a department store that is operated by a friend of Marcos' wife, Imelda.

She asked that people delay payments on their utility bills.

She said that she would speak to her followers every night at 8 p.m. by radio and asked that they follow each talk with a 15-minute "noise barrage," a Philippines form of street protest in which horns, firecrackers and trash can tops are used to make a din.

The rally broke up peacefully after more than two hours. People thronged back into the avenues of central Manila, honking horns, waving flags and flashing the campaign's laban, or fight, hand signal.