Police said up to 30,000 people, most of them young men, were converging at a major intersection along Ayala Avenue, an eight-lane artery through Manila's business district on Wednesday.
"Oust Gloria now," the crowd chanted. "Gloria is a thief."
Analysts had said anything over 20,000 marchers may signal rising public rage against Arroyo and counter her new-found resilience after a former president came to her rescue last week and the Catholic Church did not join calls that she step down.
The month-old crisis has kept financial markets nervous and raised fears that a protracted political battle will paralyse Arroyo's reforms aimed at raising revenues and cutting debt.
The president has defied demands for her resignation over allegations of vote-rigging in last year's election and graft by members of her family, saying to quit now would condemn future governments to endless turmoil.
"She will not make a decision based on the number of people in the streets," Rigoberto Tiglao, head of the presidential staff, said.
He scoffed at reports that Arroyo's foes could muster up to one million people in protests set to run until Sunday.
Some opposition groups complained that police were blocking caravans of supporters coming from the provinces.
Police said the final size of the crowd would depend on the weather and the scheduled appearance of actress Susan Roces, the widow of Fernando Poe Jr, a movie star who died in December after losing the May 2004 presidential election to Arroyo.
Police are accused of blocking
supporters coming to protest
Organisers said the protesters would swell over the day to 40,000 or 50,000 - a big step up from recent rallies of around 8000 but far from the hundreds of thousands who took to the streets of Manila to overthrow presidents in 1986 and 2001.
But analysts said Arroyo, a US-trained economist and the daughter of a late president, would not be under immediate threat unless protests grew larger and more frequent.
There were no signs yet that Arroyo's traditional foes - leftist, student and farmers' groups combined with supporters of deposed president Joseph Estrada - were being joined by the middle class, whose presence was crucial in past uprisings.
The threat to reforms prompted Moody's Investors Service on Wednesday to join the two other major ratings agencies in cutting a stable outlook for the Philippines to negative.
The military and police were on maximum alert. A police intelligence officer said anti-Arroyo forces had a war-chest of 25 million pesos ($450,000) for this week's protests.
Arroyo, who has apologised for a "lapse in judgment" for talking to an election official during vote-counting last year, has challenged her opponents to try to impeach her in Congress.
Arroyo has challenged her
opponents to try to impeach her
Her allies have said she is open to proposals to change the constitution to a parliamentary system - a scenario in which she could step down next year.
Government and opposition parties have said they are willing to work towards turning Congress into a constitutional convention that could lead to fresh elections as early as next year.
Arroyo's four-year-old presidency appeared on the brink of collapse last week after members of her cabinet resigned and political and business allies deserted.
Last-minute support from influential former president Fidel Ramos and the powerful Catholic Church's refusal to tell her to quit helped swing the momentum back in her favour but raised the prospect of prolonged uncertainty.
The military has stressed its neutrality, but rumours of unrest in the ranks are swirling in a country that has seen more than a dozen coup attempts in the past two decades.
"We are solid behind the government," said Angelo Reyes, the interior secretary and a retired military chief whose withdrawal of support in 2001 sealed Estrada's fate.
"We are solid behind the government"
Angelo Reyes, interior secretary and retired military chief
"No two situations are the same and the situation now is very different," Reyes said.
Meanwhile, the Asian Development Bank said on Wednesday it may cut off lending to the Philippines unless it speeds up reforms designed to shore up its fiscal position.
The bank said in a statement that its board had approved a new three-year lending programme under which "new lending could range from zero to as much as $1.5 billion, depending on the pace of fiscal consolidation and key sector reforms".
The ADB cannot pursue a "business as usual approach," the Manila-based bank quoted its vice president Joseph Eichenberger as saying.
The bank lent about $200 million to the Philippines last year, according to ADB spokeswoman Shamshad Akhtar.