Philippine police on Sunday arrested 17 people who burned tires on a barricade truck and hurled rocks at riot officers near the presidential palace, even as thousands of others staged largely peaceful anti-corruption rallies across Manila.

The clashes triggered a brief lockdown at the Malacanang presidential palace, where access roads were blocked by security forces. Police later said the situation was “contained” but warned that violence and vandalism would not be tolerated.

The arrested group, many dressed in black and masked, acted separately from the estimated 18,000 demonstrators who marched peacefully at Manila’s Rizal Park and along the EDSA Democracy Shrine. Later, some protesters sprayed graffiti and waved Philippine flags near the palace, prompting police to fire tear gas and make further arrests.

The rallies were fueled by public outrage over a massive flood-control corruption scandal, in which lawmakers, officials and contractors allegedly siphoned off billions of pesos from projects meant to protect poor, flood-prone communities.

“Our purpose is not to destabilize but to strengthen democracy,” Cardinal Pablo Virgilio David, head of the Catholic Bishops’ Conference, said in a statement urging calm and accountability.

President Ferdinand Marcos Jr. has acknowledged “horrible” irregularities in more than 9,800 flood-control projects worth $9.5 billion since 2022. The scandal has already forced the resignation of his public works secretary, while Senate President Francis Escudero and House Speaker Martin Romualdez also stepped down amid mounting revelations.