Opposition leader Corazon Aquino starts a campaign of civil disobedience at 3 p.m. Sunday, opening a new and highly risky chapter in her drive to become president of the Philippines.

A "people's victory" rally that convenes in a bayside park at that hour is being billed by her as the start of a campaign of nonviolent street protest that eventually will drive Marcos from power.

Aquino is convinced, as are leaders of the Philippine Roman Catholic Church and millions of her followers, that he stole the Feb. 7 presidential election from her through fraud and intimidation.

In keeping with her statements that the electoral and constitutional systems are controlled by Marcos, she is now stepping outside a system that she sees as his creation and tool. She is talking of coming to power through what would amount to revolutionary tactics, albeit with fundamentally conservative goals.

It remains uncertain, however, whether she will be able to transfer the momentum of her presidential campaign to this new undertaking, which is already making some campaign stalwarts nervous. Also in question is whether this intensely religious woman will have the nerve to lead her throngs of supporters in yellow T-shirts into situations that could provoke violence.

Sunday's rally is another milestone in the remarkable transformation of a woman who until a draft movement grew up in her neighborhood late last year seems to have entertained no political ambitions at all.

At the start, she was seen mainly as a stand-in for her husband, Benigno Aquino, the opposition leader who was assassinated at Manila airport on his return from exile in 1983. But during the past two months she has forged a drive and unity of purpose in the opposition that her husband never approached.

Looking ahead now, Aquino's people face the unpleasant reality that street campaigns that succeed in dislodging entrenched rulers are almost always ones that turn violent -- Thailand in 1973, Iran in 1979 and this month Haiti and the flight of Jean-Claude (Baby Doc) Duvalier.

In recent days, campaign workers say, Aquino and her aides have huddled with opposition politicians, businessmen and church leaders to work out a path in between, one that is peaceful but carries more punch than the street rallies that since 1983 have become a staple of Manila life -- and are ignored by Marcos.

The models offered by Mohandas K. Gandhi and Martin Luther King have been discussed. The challenge is to devise something big that will not race out of control into violence. Aquino opposes the Philippine Communists and is wary of creating a situation in which they might benefit.

In the discussions, Aquino has used the metaphor of a schoolhouse. She must find some way, she says, to capture simultaneously the approval and interest of children in the kindergarten -- that is, those who are just getting involved in her movement -- and the 8th graders.

What is now envisioned is a series of "days of protest and prayer" that would mount in passion. General strikes might be organized, although her people do not call them such. The term has become part of the Communist lexicon here, and Aquino wants the businessmen who supported her race for president to get involved too.

Other ideas include nonpayment of taxes, withdrawal of money from banks controlled by Marcos associates and refusal to advertise in progovernment media.

Together, the thinking goes, such steps would undermine Marcos' political legitimacy and, perhaps as important, his attempts to reestablish respectability in world financial markets after a debt crisis that began in 1983.

Pressure would mount irresistibly, and in the end Marcos would climb onto a plane and leave, perhaps with a shove from the United States. Aquino would be proclaimed the new president, finally collecting on the mandate she feels she won on Feb. 7.

But for many people here, this all seems too pat. The Philippine political system under Marcos, after all, has shown a remarkable capacity over the years for moving from tumult to slumber in the wink of an eye.

They hold that a good number of Aquino's people are exhausted from the campaign and are now feeling a bit foolish about the whole affair -- that Marcos actually had them believing he could be removed through a vote.

Another deterrent to her plans is fear. Having seen the type of fraud and cheating that took place on election day, people may worry that with his victory formalized, Marcos will feel free to bludgeon into submission those who continue to make noise.

In recent days, he has suggested on national television that he might prosecute opposition leaders, priests and citizen poll-watchers who he says violated the election code. A slain Aquino campaign official's funeral procession, which snaked along a 13-mile course in Manila yesterday, was a reminder of the threat of political killings.

Aquino's presidential campaign had much of the flavor of a successful church picnic. People called her "Cory." At campaign stops, society women dressed in yellow, the campaign's theme color, offered buffet lunches. Teen-aged girls ran through dances of welcome, and children in the crowd sometimes outnumbered voting-age adults.

No doubt, many such people could be counted out if things turned violent and supporting Aquino suddenly meant risking the family fortune or, at its extreme, facing gunfire on the streets of Manila.

The final uncertainty is the fiber of the leader herself. People close to Aquino say that her constant public professions of being guided by prayer and her husband's memory come from the heart. She has been slow in devising her plan, it is said, partly out of fear that it could turn bloody.

Aquino has said many times that she is willing to give her life for her beliefs. But that avowal is different from leading other people to give theirs.