Corazon Aquino, widow of slain Philippine opposition leader Benigno Aquino, today formally announced she will run against President Ferdinand Marcos in an early presidential election that was officially set today for Feb. 7.

Her announcement came as Armed Forces Chief of Staff Gen. Fabian C. Ver, reinstated after he was acquitted a day earlier of conspiracy charges in the assassination of Aquino's husband, began a high-level shake-up in the military.

Later today, to protest the acquittal of Ver and the 25 other defendants, a few thousand Filipinos staged a noisy demonstration for several hours in various parts of the city. Motorists honked their horns, gangs of youths banged on tin cans and steel road railings, and burned tires at busy intersections. About 1,500 youths on one boulevard stoned a fire truck while its crew was trying to extinguish a fire.

Aquino, 52, confirmed that she asked Salvador Laurel, 57, the other main opposition contender, to be vice president on her ticket. Laurel has not given his answer. But there has been a perceptible softening on both sides to accepting an Aquino-Laurel ticket.

Opposition sources say privately that they have been pressured to unite not only by their own supporters but also by Cardinal Jaime Sin, the archbishop of Manila and senior prelate here, as well as by U.S. officials. Most analysts here give the opposition no chance of winning in the presidential election if the anti-Marcos vote is divided.

At a press conference with 200 of her supporters cheering "Cory, Cory," Aquino said: "I hereby affirm my candidacy and confirm my willingness if elected to serve our people as president of the Republic of the Philippines."

Aquino made her announcement hours after Marcos signed into law a bill authorizing the election on Feb. 7, with the campaign to begin on Dec. 11. Opposition members of the legislature petitioned the Philippine Supreme Court to declare the bill unconstitutional because Marcos has refused to resign before running.

In the military reorganization, the state news agency reported that Ver replaced Navy chief Adm. Simeo Alejandro with a friend, Coast Guard Commodore Brillante Ochoco, and also named a new Coast Guard commander and reassigned or shuffled officers in five Army and Navy commands.

Ochoco has criticized a reform movement in the military that was initiated by disgruntled midlevel officers. They say they want to halt what is seen as growing demoralization in the armed forces and to push for the retirement of generals who are beyond retirement age.

Gen. Fidel Ramos, who had been acting chief of staff while Ver was on leave, is back in his former position as deputy chief of staff.

Marcos has not made clear how long Ver's reinstatement will last. But some political observers predicted that Ver would move in his men before stepping down. The news agency said there would also be changes in 20 provincial commands.

Earlier today, Marcos met for four hours with top military and defense officials to discuss recommendations by Ver. A government announcement said Marcos had ordered the other 24 acquitted military personnel to return to their posts. In that announcement, Marcos said he would retire or reassign about 50 senior officers and create six new combat battalions to improve the military's counterinsurgency efforts.

In her press conference today, Aquino indirectly criticized Laurel, who is often seen as representing traditional politics. Without referring to him by name, she said: "The challenge [facing the opposition] is to be able to field a candidate who cannot, rightly or wrongly, be seen as a continuation, potential or actual, of the Marcos regime."

Aquino, asked about a New York Times interview of Imelda Marcos, in which the first lady disparaged her, answered: "For the time being I will allow Imelda Marcos to talk all she wants. But when the campaign starts I will give my answer."

Imelda Marcos is seen as a probable running mate for vice president with her husband.

In response to charges by Marcos that she does not have the necessary political skills or experience to run a government, Aquino said: "For 20 years we have had one of the most brilliant Filipinos. Look what happened to our country. It has been devastated. I give the 1.2 million Filipinos [who signed a petition endorsing her candidacy] the hope that I can bring about the necessary change that we are looking for."

Asked if she would be manipulated by her advisers, she said: "People who know me know that I am pretty stubborn. While I am advised by many people, in the final analysis it will be me who will make the decision."

She said her first priority if elected president will be to reinstate the writ of habeas corpus and release all political prisoners. She also said she wanted a new constitution to be written. The opposition has accused Marcos of consolidating his rule by decree by adopting highly controversial amendments to the constitution.

The most important issue in her platform, she said, is credibility. She said she has to inspire enough confidence to bring in local and foreign investments. Capital flight from the Philippines in the period since the assassination of her husband led to the current instability and economic crisis.

Aquino reiterated that she will respect the agreement with the United States on the two large military bases here until the lease expires in 1991. But she refused to say whether she will allow the bases to remain.

She also said she was for nonviolence and wants the Communist rebels to denounce violence.