Former President Corazon Aquino, a key leader of uprisings in the past, called on Filipinos to support efforts to impeach President Gloria Macapagal Arroyo.
"It is good for us to show our support to the congressmen by being out there and marching," Aquino said at a news conference to announce the launch of the Coalition for the Truth, an alliance of civil society and opposition political parties.
"Maybe this is one way of telling them how much it means, not only to me but to all of us who love our democracy and who are searching for the truth, " Aquino said.
The protesters will march on Tuesday to the House of Representatives.
"We all seek the truth ... and yet ... every avenue for arriving at the truth has been blocked, and every opportunity to find the truth is being closed," said a coalition statement read by Brother Armin Luistro, president of De La Salle University, an upscale Roman Catholic school.
"Gloria Macapagal Arroyo's response to our call for the truth has been to suppress evidence, hide her accomplices, engage in grand cover-up, sow fear, foment distrust, and use every instrument at her disposal to encourage division among the people," the statement said. "We say with one voice, Gloria Macapagal Arroyo must go."
Joining Aquino in the coalition are Susan Roces - the widow of Arroyo's main presidential rival Fernando Poe Jr. - and popular televangelist Eddie Villanueva.
Asked by a reporter whether the protest would have supporters within the military, Aquino said her call was for nonviolent action. But she pointed out that that she was once the country's commander-in-chief and still has friends among the military.
Villanueva, head of the religious group Jesus is Lord Movement, said that the impeachment process was "the last chance for us to show that peaceful, constitutional process is doable," warning that if it fails "it will be difficult to stop the people from going to the streets and to exercise their sovereignty."
But Cabinet Secretary Ricardo Saludo told ABS-CBN television: "They are not asking for the truth, they just want the President to go."
Philippine police were on alert ahead of expected anti-Arroyo protests and a stormy debate by lawmakers who are to decide this week on the fate of impeachment charges against her.
Arroyo's allies on the House of Representatives' justice committee voted last Wednesday to throw out all three impeachment complaints against her in a session that was boycotted by opposition lawmakers who have accused their pro-Arroyo counterparts of railroading the decision.
The 236-seat House, which is dominated by Arroyo's allies, was expected to vote this week on whether to accept or reject the committee's decision to throw out the impeachment charges.
Arroyo is accused of rigging last year's election and of bribery, corruption and other crimes. She has denied any wrongdoing and has refused to resign, the AP reports.