Thousands of people have joined a protest in the Philippine capital, Manila, to demand the resignation of President Gloria Arroyo.
Corazon Aquino and Joseph Estrada both appeared before the crowd
The protesters want her to step down over corruption claims surrounding her husband and an ally's involvement in a government deal with a Chinese firm.
Both men deny the allegations and the deal has been cancelled.
Former presidents Joseph Estrada and Corazon Aquino joined the demonstration as security forces went on high alert.
Ms Aquino, 75, once a staunch ally of President Arroyo, climbed on a stage to address the crowd, which police estimated to be 15,000-strong, although organisers said there were more.
The protest was the biggest since the corruption claims emerged
"I thought my work was done because I am already old," she said. "But this is what the times ask for, for us to unite so that the deceit will end and we will find out the truth.
"Thankfully there are still many of us shouting, Gloria, enough, resign already."
Mr Estrada, pardoned by President Arroyo last year after being sentenced to life in prison for corruption, joined his former rival on stage.
Correspondents say it was the largest demonstration against the president since the allegations emerged weeks ago.
The corruption row relates to a multi-million dollar deal for a government broadband network and claims Ms Arroyo's husband and an ally sought illegal payments.
Ms Arroyo cancelled the deal because of the claims, which were being investigated by the Senate.