Philippine police say the number of suspected drug dealers killed during President Rodrigo Duterte's crackdown has risen to more than 500 in just over a month, sparking protests and plans for a Senate investigation.

The national police's Directorate for Investigation and Detective Management said in a report Thursday that 525 drug suspects have been killed in clashes with police and more than 7,000 have been arrested. It said more than half a million others have surrendered to authorities.

Left-wing activists protested against the killings and Senate Justice Committee chairwoman Sen. Leila de Lima said it will start an investigation into the killings on Aug. 22.

Duterte has promised to end the illegal drug menace and other crimes in three to six months.