But a day of largely peaceful mass protests erupted into violence as riot police deployed water cannon in clashes with scores of mostly young masked people who hurled rocks and shattered the glass of one police outpost.

Police arrested 72 people -- including 20 minors -- in two separate incidents that saw at least 39 officers injured and a trailer that was being used as a barricade set ablaze, according to a spokeswoman.

Major Hazel Asilo told AFP it was unclear if those arrested were "protesters or just people who are causing trouble".

Rage over the so-called ghost infrastructure projects has been mounting in the Southeast Asian country since President Ferdinand Marcos put them centre stage in a July state of the nation address that followed weeks of deadly flooding.

Marcos said early last week he did not blame people for protesting "one bit".

Sunday in the capital began without violence with a morning demonstration at a park that drew nearly 50,000 people, according to city estimates.

Thousands more joined an afternoon rally at the capital's EDSA thoroughfare, ground zero for the 1986 movement that ousted Marcos's dictator father.

- 'This is enough' -

"It's very rare for me to go to rallies, but this situation was bad enough that I was really urged to say 'this is enough'," Mitzi Bajet, a 30-year-old designer told AFP at the EDSA protest.

Teddy Casino, 56, chairman of left-wing alliance Bagong Alyansang Makabayan, said his group was demanding not only the return of stolen funds, but also prison time for those involved.

"Corruption requires people to go to the streets and express their outrage in the hope of pressuring government to actually do their jobs," he said.

Renato Reyes, another organiser of the first protest, later said he had been hit in the face with a rock as he tried to leave an area near the presidential palace.

"They could be provocateurs or they could just be really angry at what is happening," he said of the masked protesters, adding the government could not "ignore the problem of corruption".

Some of those at the clashes could be seen holding aloft the pirate flag seen at recent Indonesian protests sparked by low wages, unemployment and anger over lavish lawmaker perks that left at least 10 dead.

An AFP journalist at the scene witnessed police throwing rocks back towards protesters, something Asilo, the spokeswoman, denied.

- Billions lost -

The Department of Finance has estimated the Philippine economy lost up to 118.5 billion pesos ($2 billion) from 2023 to 2025 due to corruption in flood control projects.

Greenpeace has suggested the number is actually closer to $18 billion.

Earlier this month, the owners of a construction firm accused nearly 30 House members and Department of Public Works and Highways (DPWH) officials of taking cash payments.

The scandal has already sparked leadership changes in both houses of Congress, with House speaker Martin Romualdez, a cousin of Marcos, tendering his resignation earlier this week as an investigation got underway.

On Sunday, multiple politicians were among those taking part in the EDSA protest, an event supported by the powerful Catholic Church that drew numerous families.

"This is not partisan," said 58-year-old Manuel Dela Cerna, who said he had attended the People Power protests at EDSA four decades earlier.

"They are draining the people's money while citizens suffer from floods, their homes being swept away, while officials ride private planes, live in mansions," he said.

The Philippines has a long history of scandals involving public funds, in which high-ranking politicians found guilty of corruption have typically escaped serious jail time.