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.

On Monday, Marcos said he did not blame people for protesting "one bit" while calling for demonstrations to remain peaceful. The army has been placed on "red alert" as a precaution.

"There were times I personally waded through floods," Aly Villahermosa, a 23-year-old nursing student from Metro Manila told AFP as an estimated crowd of 13,000 gathered Sunday morning in the capital's Luneta Park.

"If there's a budget for ghost projects, then why is there no budget for the health sector?" she said, calling the theft of public funds "truly shameful".

Teddy Casino, 56, chairman of left-wing alliance Bagong Alyansang Makabayan, said the group was demanding not only the return of stolen funds but 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.

Even bigger crowds are expected to gather later in the day to march down the thoroughfare known as EDSA, site of the People Power Movement protests that helped oust Marcos's dictator father from power in 1986.

The flood control 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.

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 Department of Finance has estimated that 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.

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.

On a recent visit to Bulacan, a flood-plagued province north of Manila where multiple bogus projects have been identified, AFP reporters saw residents trudging through murky water in rubber boots.

Elizabeth Abanilla, an 81-year-old retiree, said politicians and contractors were equally guilty.

"They should not have handed (money) over before the job is completed," she said. "Both of them are guilty."