They also condemned the attempt by her allies in the House of Representatives to amend the Constitution by calling a constituent assembly.
The left-wing activists said they had not announced the rally so that police would not block them from staging the rally, which snarled traffic already heavy because of the approaching Christmas holiday.
About 30 policemen arrived at the scene, but showed no sign they would break up the rally.
Occupying the intersection of Ayala and Paseo de Roxas Avenues, the protesters waved small Philippine flags and raised placards reading "Be not afraid" and "Oust Arroyo now."
"If we can, we want to camp out here until Arroyo steps down," said Virgie Pinlac of the Movement for National Democracy, one of several groups involved in the protest.
"It’s almost Christmas, but GMA continues to commit wrongdoings and we’ll continue to protest," Pinlac said.
The protesters said they joined a Roman Catholic church-led prayer rally at Rizal Park last Sunday, but were not allowed to unfurl their streamers or deliver political statements, prompting them to stage another demonstration.
Catholic bishops organized Sunday’s rally to fight plans by Arroyo’s allies to revise the 1987 Constitution. Her camp backed off last week, diffusing tension and turning the rally largely into an opposition show of force.
Makati City police chief Superintendent Gilbert Cruz said the rally grew too fast and it was too late for them to stop it.
Among those who joined the rally were Bishops Deogracias Iñiquez and Antonio Tobias, former vice president Teofisto Guingona, and Representatives Allan Peter Cayetano and Riza Hontiveros-Baraquel.
Former Social Welfare and Development Secretary Corazon "Dinky" Soliman, Partido ng Masa partylist Representative Renator Magtubo, and other groups like Sanlakas and Laban ng Masa were also there. — With AP