Authorities warned against rowdy protests in an effort to pre-empt a repetition of "people power" revolts that have ousted two presidents in the last two decades. But left-wing and opposition groups ignored the warning, and gathered along Ayala Avenue, a key thoroughfare in Manila's Makati financial district, calling for Arroyo's resignation.
"She's digging in," said opposition Makati Mayor Jejomar Binay, who led the protests. "But we will not stop until she steps down."
The flag-waving crowd, chanting "Arroyo resign" to the beat of disco music, grew to about 5,000 and then began dispersing at nightfall. Some marched toward a "people power" commemorative shrine to join an anti-Arroyo group holding a prayer vigil.
Arroyo has been buffeted by almost daily protests since allegations of electoral fraud surfaced last month, coupled with accusations that her family took illegal gambling payoffs.
But they have not matched the huge demonstrations that toppled the late dictator Ferdinand Marcos in 1986 and Arroyo's predecessor, Joseph Estrada, in 2001.
Interior Secretary Angelo Reyes, among Cabinet members loyal to Arroyo, said police were under orders to maintain order.
"The entire police force will see to it that no vested interest group or personal ambition will take advantage of this situation to promote their own ends," he told reporters. "We will see to it that there is no chaos, there is no turmoil, and that peace and order will reign."
Several key players in the two "people power" revolts, including Marcos' successor, Corazon Aquino, frowned on the prospect of a similar uprising against Arroyo because of fears it was getting out of hand.
At least 10 Cabinet members, Aquino and business groups asked Arroyo to resign and hand over power to her vice president, Noli de Castro, but rejected any attempt to oust her by force. A defiant Arroyo said she would not quit but was ready to face impeachment complaints before Congress.
The 115,000-strong military, which played a key role in both uprisings, declared neutrality.
Military officials said they were on guard against possible attacks by communist guerrillas at the height of the political storm.
Two former military rebels have warned that junior officers have organized a group plotting to oust Arroyo and hand over power to a caretaker government that would bring back political and economic order until new leaders were elected.
"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. "The strength of your leadership is on the line. This is not the time for weakness."
The Philippines, which has a recent history of coup attempts, has been swamped by rumors of military unrest despite official denials.