Some 30,000-35,000 people packed the Makati financial district on Wednesday to demand the president quit over claims she stole the 2004 election, but the turnout was lower than the organisers had hoped for.
"I don't think you are going to get her out with all of these rallies that we are holding," said senior opposition Congressman Ronaldo Zamora.
But Zamora said they might have enough numbers to vote for impeachment, because some of Arroyo's allies in Congress had changed sides.
"Basically, for the first time, the numbers are moving towards our direction. We will be starting the impeachment process with a really substantial number," he said.
Arroyo ally, Congressman Joey Salceda, quoted in the Philippine daily, Inquirer, said the opposition was already assured of 73 of the required 79 votes to hear the impeachment complaint.
The turnout in Wednesday's rally
was lower than expected
Her spokesman, Ignacio Bunye, said street protests would not solve the country's myriad problems.
"We respect everyone's right to freedom of speech and assembly, but we still maintain that the problems besetting the country will not be solved by loud noises in the street," Bunye said.
Share prices closed 1.37% higher on Thursday after political uncertainty subsided.
"It looks like yesterday's rally, though exceptionally well produced, did not get the numbers. It has somehow eased the uncertainty that Mrs Arroyo would just walk away because of a mass uprising," said Nestor Aguilao of DA Market Securities.
Arroyo vowed that street protests would not drive her from office.
"If they want unrest in the street, I will enforce the law. I will stop any violence.
Arroyo says she will not resign
despite the protests
"If they want to subvert the constitution, I will fight them every step of the way," she said in a speech aired over government radio.
"This is not a popularity contest, this is a contest for improving the life of our population. My [opinion] surveys may be down, but my resolve has never been higher."
The military and police announced that they had lowered the red alert ordered in the capital for the rally, which passed off peacefully.
However, Executive Secretary Eduardo Ermita said the government was anticipating an even larger protest rally on 25 July when the president makes her annual address to Congress.
Zamora said the opposition was strengthening an impeachment complaint previously filed by an activist lawyer and would present it to the House of Representatives by 22 July.
At least one-third of members of the House must vote in favour of impeachment before the case goes to the Senate for trial.
The opposition charges that an audiotape of a wiretapped conversation shows Arroyo cheated to win the elections. In it, a woman sounding like Arroyo holds a conversation with an election commissioner, who appears to offer to fix her vote.
Arroyo has denied any wrongdoing and refused to resign even after key cabinet members and political allies turned against her.
She has welcomed any effort to impeach her, saying this will give her a chance to clear herself.
"I don't think you are going to get her out with all of these rallies that we are holding"
senior opposition congressman
The opposition was initially wary of impeachment for fear that Arroyo's allies, who control Congress, might quash the complaint.
"Essentially, the president made up the minds of both the majority and the minority. She has made it very clear that she is not resigning," Zamora said.
Opposition Congressman Peter Cayetano said the impeachment complaint was not for specific crimes, but rather for "obstruction of justice and for perjury".
The impeachment procedure in the House would take at least six months, assuming no further legal issues are raised, Cayetano added.
House Majority floor leader Prospero Nograles said talk about how congressmen would vote was "speculative".
"I think this is going to be a matter of conscience. The entire Filipino people will be watching you (legislators)," he said.
"I don't think [the congressmen] will fool around with this. They will look at the evidence, look at the political situation, consult with their constituents."