President Gloria Macapagal Arroyo declared a state of emergency Friday, saying she had quashed a coup plot but the Philippines still faced a "clear threat" from treasonous forces.

Clashes erupted as protesters marched on the eve of the 20th anniversary of the culmination of a four-day "people power" uprising that ousted dictator Ferdinand Marcos.

Riot police used water cannons to disperse about 5,000 protesters defying a ban on rallying at a shrine to the uprising on a major highway in the capital.

Police used truncheons and shields to roust a stone-throwing group trying to gather for a second protest in the same area. Several people were arrested; others were bloodied.

Former President Corazon Aquino and about 5,000 people were later allowed to march peacefully in the financial district to a memorial to her late husband Benigno, whose assassination in 1983 sparked massive protests that led to the revolt against Marcos.

The protesters want Arroyo resign because of alleged election-rigging during the 2004 polls, as well as corruption and alleged human rights abuses such as the alleged killing of activists by security forces. Arroyo vigorously denies the accusations.

Amid a massive security clampdown, the military barricaded its camps to keep troops from joining the demonstrations and detained an army general allegedly involved in the takeover plot. The military has played major roles in two "people power" revolts and has a recent history of restiveness.

While she vowed she was in control, Arroyo clearly was worried about losing her grip on events as her opponents tried to hijack the anniversary activities.

Many Filipinos see the four-day "people power" revolt that toppled Marcos on Feb. 25, 1986, as their country's proudest moment.

Presidential spokesman Ignacio Bunye told reporters that commemorations have been canceled and that the military was ordered "to prevent and suppress lawless violence."

Arroyo, who survived two earlier coup attempts, said the political opposition, along with extreme elements of the political left and right, were determined to bring down the elected government.

"I am declaring a state of emergency because of the clear threat to the nation," a defiant Arroyo said in a taped, nationally televised statement.

"This is my warning against those who threaten the government: the whole weight of the law will fall on your treason."

Appealing for calm, she claimed the military had quashed a coup plot by some military officers and their men.

"There were a few who tried to break from the armed forces chain of command, to fight the civilian government and establish a regime outside the constitution," said Arroyo, who held a pre-dawn emergency meeting of her national security council. "We crushed this attempt."

She stopped short of declaring martial law, a sensitive issue because Marcos used it to rule by decree.

Her chief of staff, Mike Defensor, said no curfew will be imposed but the declaration bans rallies, allows arrest without warrant, permits the president to call in the military to intervene and lets her take over facilities — including media outlets — that may affect national security.

Arroyo's aides linked former President Joseph Estrada and several others to the plot.

Estrada, who was toppled in massive street protests in 2001 and held under house arrest on charges of plunder, dismissed the allegations.

He said he's been out of work and under detention for five years and didn't have the money to finance a coup. "I don't have any work, how can I finance?" he asked.

Aquino, a one-time Arroyo ally, criticized the emergency declaration and reiterated a call for Arroyo to "make the supreme sacrifice" and resign.

"I believe that during these times, we should not forget that many sacrificed to regain our democracy," Aquino said. "We cannot just keep quiet because that is what happened during martial law. Our dictator then believed that he can do anything to keep himself in power."

The opposition said the declaration showed the government's desperation.

The Integrated Bar of the Philippines, the largest lawyers' group in the country, said it will question the legality of Arroyo's declaration before the Supreme Court, according to its president, Anselmo Cadiz.

"It could result in more political hemorrhage and security risk," said Rep. Roilo Golez, Arroyo's former national security adviser, who withdrew support from her. "This could get out of control ... if her crisis team doesn't manage this well."

Rep. Teodoro Casino, a leftist leader, said anti-Arroyo protests won't end.

The Philippine stock market and the peso both plunged.

U.S. State Department spokeswoman Janelle Hironimus said: "We are monitoring the situation carefully. We firmly support the rule of law and constitutional government. Violence should be avoided."

Military chiefs said they backed Arroyo. They arrested an unspecified number of people, including an army general who leads an elite special forces unit, for alleged involvement in a coup plot.

"We have reduced the threat," army chief of staff Gen. Generoso Senga said. "We cannot say that it has been stopped."

Police already were on heightened alert nationwide as widespread reports of a coup plot have been circulating for more than a week; even elementary school students were discussing it in detail.

Army chief Lt. Gen. Hermogenes Esperon has said the coup plot involved forcibly removing Arroyo, establishing a revolutionary government and abolishing "democratic institutions."

Arroyo survived three impeachment bids in September, when her dominant allies in the House of Representatives used a technicality to block complaints of alleged massive corruption and vote-rigging.