Opposition lawmakers in the Philippine parliament filed impeachment proceedings Monday against President Gloria Arroyo (search), accusing her of rigging last year's vote by speaking with an election official before a final tally was announced.

In her annual state of the nation address hours later, Arroyo made no direct reference to the allegations, but she urged Congress to change the constitution to shift to a parliamentary form of government that could ease the country's constant political instability.

Arroyo, looking tired with bags under her eyes after dealing with a 2-month-old scandal, referred indirectly to the two "people power" revolts that have forced out two presidents since 1986 and the constant rash of coup attempts and takeover rumors.

She said the Philippines had become too polarized to make the changes necessary to meet the challenges of the 21st century.

"The system clearly needs fundamental change — and the sooner, the better," said Arroyo, who has come under pressure to quit with nearly five years left in her term. "Ours is a country divided. One is a Philippines whose economy ... is now poised for takeoff. The other ... has become a hindrance to progress."

A parliamentary system — where lawmakers can vote out a sitting government — would ease the upheaval that accompanies ousting a leader who has lost the ability to govern. But if the changes are approved by a referendum, the next step would likely be fresh elections which could cut Arroyo's term short.

Police and the military were on alert as about 25,000 opposition supporters gathered outside the House building in suburban Quezon City (search) where Arroyo spoke, demanding her resignation. About 5,000 supporters of the president rallied in support of her nearby.

Some 6,000 police, along with a contingent of anti-riot air force personnel, provided a security cordon around the building.

All major roads leading to the building were blocked and traffic rerouted. Security of the House was reinforced with steel containers, concrete blocks and barbed wire. With the roads tangled, Arroyo flew in by helicopter.

The impeachment complaint claims Arroyo "cheated and lied" to obtain and hold power.

"By so flouting justice and the rule of law, she has committed an unforgivable outrage against the Filipino people," the complaint says.

Arroyo has denied manipulating the May 2004 ballot by discussing vote counting with an election official before she was declared the winner. She said she's ready to face an impeachment trial to clear her name, and she announced that a "truth commission" will probe the allegations against her.

Her aides have moved to block the impeachment complaint on a legal technicality.

About 40 left-wing and opposition lawmakers broke into applause and raised clenched fists after filing the impeachment complaint in Congress. Some opposition lawmakers boycotted her address.

"Those guilty of rigging the impeachment process should be held fully responsible for the highly likely political firestorm," Rep. Roilo Golez said, citing media reports that some lawmakers were being offered bribes to shun the impeachment bid.

Left-wing lawmakers have warned of a "people power" revolt — the type of street demonstrations that ousted late dictator Ferdinand Marcos (search) in 1986 and President Joseph Estrada (search) in 2001 — if pro-Arroyo lawmakers kill or weaken the complaint. Arroyo supporters have a solid majority in the House of Representatives.

The first revolt that ousted Marcos was widely hailed, but the one that forced out Estrada was criticized for setting the bar too low to throw out an elected government.

The impeachment complaint must be endorsed by at least one-third of the 236-member House to be sent to the Senate for trial.

Opposition Rep. Alan Peter Cayetano said late Sunday the complaint was a few names shy of the required 79 signatories. But he said he was confident the opposition would gather the rest in a few days.

Otherwise, the complaint would be referred to the House justice committee, where it could be watered down.