With the opposition warning that dropping the charges could worsen a crisis on Tuesday, former president Corazon Aquino urged Filipinos to back the impeachment by joining a protest march.
"Maybe this (marching) is one way of telling them how much it means ... to all of us who love our democracy and who are searching for the truth," Aquino said on Monday at a news conference on the launch of the Coalition for the Truth, an alliance of civil society groups and opposition political parties.
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.
"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.
"We say with one voice: Gloria Macapagal Arroyo must go," the statement said.
The protesters were set to march to the House of Representatives on Tuesday.
"Every avenue for arriving at the truth has been blocked, and every opportunity to find the truth is being closed"
De La Salle University president
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 protests.
Opposition lawmakers warned Congress on Monday that dropping impeachment charges against Arroyo could spark unrest and worsen a crisis over claims that she rigged elections.
The 236-seat House of Representatives - overwhelmingly dominated by pro-Arroyo legislators - started a final debate before voting on whether to uphold a justice committee's decision to throw out all three impeachment complaints.
About 140 pro-Arroyo lawmakers met on Monday before the debates began and reaffirmed support for the president.
"To the last man ... they have decided that they will not join the move to impeach the president at this time because it's not going to do the country any good," said pro-Arroyo legislator Prospero Nograles.
Arroyo's opponents say the impeachment process was the last legal avenue to press her to answer the charges and close the crisis.
Even moderate lawmakers have warned that suppressing the charges against Arroyo in Congress could spark another "people power" revolt like those that ousted two presidents in the past two decades or inspire the military to intervene.
"This is the last chance for us to pave the rule of law and the constitutional process," said opposition spokesman Representative Alan Peter Cayetano.
Arroyo has faced persistent calls
"We're saying we can't stop the people," he said. "As far as we opposition congressmen are concerned, we've done
our job, but what happens after that is beyond us."
Anti-Arroyo lawmakers need at least a third, or 79, of the legislators to send the impeachment complaints to the Senate for trial. They have said they are a few signatures short.
However, a key Arroyo backer, Representative Marcelino Libanan, said the impeachment charges could no longer be resurrected because the justice committee had already dismissed them.