When Corazon Aquino formally filed to run for president of the Philippines on Dec. 11, in the space marked "occupation" on her certificate of candidacy she wrote "housewife." The 53-year-old widow and grandmother was given virtually no chance of unseating President Ferdinand Marcos, 68, a master politician who had ruled this country for 20 years.

In the 2 1/2 months since then, she has undergone a remarkable transformation from election campaign underdog, to odds-on favorite to win by a landslide, to post-election street demonstration leader and, finally, to the president of a provisional government and first woman president of the Philippines.

She has emerged as a charismatic national leader unprecedented in modern Philippine history.

From the start, her prime appeal to Filipinos was that she was not a politician in the traditional mold. Rather she had a spiritual focus embodying moral authority and her outlook reflected a disgust with Marcos and his two decades of authoritarian, corrupt and often brutal rule.

She began her quest last November as a stand-in for her husband, opposition leader Benigno Aquino, who had been assassinated in 1983. She repeatedly accused Marcos publicly of having ordered the assassination, but a court acquitted 25 military men and one civilian of the murder.

Since then, she has developed enormous political skill and achieved a goal that her husband -- once touted as a future president -- was never able to achieve.

Aquino is known simply as "Cory" here. She begins her term with a seemingly limitless supply of good will from Filipinos, who apparently feel her lack of experience will be overcome through common sense and honesty.

Her speeches and actions indicate that she holds not a trace of doubt that right lies on her side, an attitude that could lead to tension in dealing with the criticism that is certain to emerge after today's euphoria wears off.

Although Aquino has come to power through revolutionary means, she is essentially a conservative product of a privileged upbringing in one of the country's wealthiest land-owning families. She was educated in the United States and retains something of an American twang when she speaks English.

Aquino was born Corazon Cojuangco in 1933, the sixth of eight children. After attending high schools in Philadelphia and New York, she graduated from Mount St. Vincent College in the Bronx with a degree in French and mathematics. She studied law in Manila but gave it up after meeting her husband, who later became the youngest governor of Tarlac province and the youngest member of the Philippine Senate.

In September 1972, Marcos declared martial law and the first man he had arrested was Benigno Aquino. He was jailed on sedition and murder charges and sentenced to die. But he was allowed to go the United States for heart treatment in 1980 under pressure from the Carter administration, and Corazon and their five children accompanied him.

She was known as a political wife, serving coffee while her husband talked endlessly of politics with visitors. It was characteristic that she did not accompany him when he returned to the Philippines in 1983 and was shot dead at Manila International Airport.

The widow was a frequent sight in the demonstrations and protests that eruped after Aquino's death. It was only in 1985, as Marcos talked of an early election, that her name began to be mentioned as a possible contender, as the one person who could unify the country's perenially divided opposition.

"What do I know about being president?" was her response at the time.

But when Marcos called the election for Feb. 7, pressure mounted and it was then that her skills as a politician began to appear.

In back-room bargaining that ran right up to the filing deadline, she resolved a rivalry that threatened to produce two opposition presidential contenders and hammered out a deal in which she would run as president, with Salvador Laurel, head of the largest opposition party, as the vice-presidential candidate.

Aquino stumped tirelessly across the country for the two-month campaign, hitting cities and villages in 68 of the country's 74 provinces with "whistle stop" motorcades that drew enormous crowds.

With voting completed, she seized the initiative within hours, declaring herself the winner and putting Marcos on the defensive as the parliament that his party controlled came up with a count showing him the victor by 1.5 million votes.

Aquino then declared a campaign of strikes and economic boycott, and, again, acted without hesitation when two military leaders launched a rebellion Saturday aimed at toppling Marcos. She immediately declared support for it and called on her followers to join "people's power" protest in which thousands of citizens gathered in strategic locations to prevent counteraction by loyalist Marcos troops.

Tuesday night, the struggle ended when Marcos and his family boarded U.S. helicopters in the palace grounds and flew away. "The long agony is over," Aquino told a television audience.

The core of Aquino's program of government is cleaning up the corruption and favoritism that characterized the Marcos era and ending restrictions on civil rights that began with his declaration of martial law in 1972.

Speaking after her swearing-in Tuesday morning, she said: "It is fitting and proper that as the rights and liberties of our people were taken away at midnight 14 years ago, the people should formally recover those rights and liberties in the full light of day."

For dealing with the Philippines' Communist insurgency, now reported to have 15,000 people under arms, she has proposed a six-month cease-fire and an offer of amnesty. She has said she would follow up with military force against any who did not comply.

She contends that most insurgents are not real Communists, just disillusioned people looking for an alternative to Marcos, which she says she has now provided.

Aquino has said that she will allow an agreement that gives U.S. forces access to Clark Air Field and Subic Bay Naval Base, the largest U.S. overseas bases, to continue until it expires in 1991. She has refused to say what might happen to the agreement after that.

She has hinted, however, that change could follow, saying no country can allow a foreign power open-ended control of a part of its territory. But many analysts feel that the employment the bases provide, the role they play in the Asian balance of power and her basically conservative bent will lead her to let them stay.

Her aides say her economic policy will be heavily free-market oriented. She contends that government intervention and favoritism shown to Marcos "cronies" are largely responsible for the economic and debt crisis in which the Philippines now finds itself.

Gathered around her is a corps of advisers that includes leftists, Jesuits and successful bankers and businessmen dubbed the "shadow presidents" by Marcos during the campaign. They are expected to play an important role in governing.

But Aquino denies that she will be a passive president. "I have already proved that I cannot be manipulated by anybody," she said in a recent interview.

Marcos supporters charged that an Aquino government would mean chaos and creeping communism. They said Aquino secretly sympathized with the insurgents and had surrounded herself with Marxists.

But among the middle class, there are those who feel that the more serious danger is that she will emerge as another oligarch, a president who puts the interests of her extended family and its vast holdings above those of the nation.

She has said she will work at the opulent Malacanang presidential palace but not live there. With fears for her security likely to persist, it is not clear whether she will continue to live at her home in a middle-class neighborhood in Quezon City, a Manila suburb.