MANILA, Philippines -- Gloria Macapagal Arroyo's presidency was in tatters Friday, her base of support eroding by the hour, as former backers abandoned the Philippine leader and added to calls for her resignation over an election scandal.

But in a radio address as thousands of people protested against her in Manila's financial district, Arroyo defiantly vowed not to quit, criticized those urging her to resign _ including former President Corazon Aquino, a close ally _ and said she was forging ahead by naming a new Cabinet.

"The Philippines has fallen into a dangerous pattern, where the answer to every crisis is to subvert due process rather than work within the system," Arroyo said. "This must stop. I was duly elected to uphold the constitution and ensure that the institutions of the nation were strengthened, not weakened."

Despite her effort to present a business-as-usual approach, Arroyo appeared to be barely holding on, with prospects rapidly fading for anything other than a peaceful handover of power to Vice President Noli de Castro, who leftist groups said must show that he's not tainted, too.

A third of her Cabinet quit Friday, telling a news conference that Arroyo has been crippled by allegations that she rigged last year's presidential race and has lost the ability to lead. She has denied that she rigged the elections.

The Liberal Party, a key part of the ruling coalition backing Arroyo, also called for her resignation and said it would support an impeachment process if she won't yield power.

Two major business groups, the Makati Business Club and the Financial Executives Institute of the Philippines, also jumped on the bandwagon, as did a coalition of nongovernment groups that backed her ascension to the presidency during the 2001 "people power" revolt that ousted predecessor Joseph Estrada.

Even Aquino, one of Arroyo's closest supporters and the beneficiary of the first "people power" uprising that forced out dictator Ferdinand Marcos in 1986, said it was time to go because the country can't afford another uprising.

"I say that their actions cause deep and grievous harm to the nation because they undermine our democratic principles and the very foundation of our constitution," Arroyo said. "Once again, we're ... perpetrating a system that is broken and will remain broken until fundamental reforms are put in place."

Arroyo repeated her offer to undergo an impeachment process, telling her opponents they should follow proper procedure and take their case to Congress, where her administration holds majorities in both houses.

"In the meantime, I will continue to focus on the people's business, which is getting our economy moving and creating better quality of life for our people," Arroyo said.

The remaining two-thirds of Arroyo's Cabinet held a news conference supporting her. Still, even those allies were essentially shadows, handing in the courtesy resignations she sought Thursday night to give her a free hand in trying to navigate her way out of her biggest crisis.

For the time being, the influential Roman Catholic Church and the powerful military _ also players in the 2001 revolt _ were on the sidelines, but bishops were to meet through the weekend, and the military hardly gave a ringing endorsement, only saying it would resist any moves outside the rule of law.

"Commanders should act swiftly against any behavior that challenges or breaks away from the chain of command," military chief of staff Gen. Efren Abu said in a statement. "This is not the time for weakness."

The capital's police force went on full alert and additional contingents were securing the presidential palace.

The leftist group Bayan urged stepped-up street protests, which so far have been a fraction of the size of the two "people power" revolts. Coup rumors have been circulating.

Joseph Mussomeli, the U.S. Embassy's deputy chief of mission, said Washington firmly opposes any forced change of power _ "anything that will weaken the institutions that we genuinely believe are the true safeguards for your own liberty."

The backbone of Arroyo's economic team _ her economic, trade and budget secretaries _ were among the eight government ministers and two other members of her Cabinet who abandoned her.