Philippine police say simultaneous anti-drug operations in a northern province have left 21 alleged drug offenders dead. Officials say it is the highest death toll in a single day since President Rodrigo Duterte launched his 'war on drugs' in July last year.

Senior Superintendent Romeo M. Caramat Jr. said Tuesday that operations in Bulacan province in the past 24 hours left 21 dead and 64 others arrested. Police say the suspects had offered armed resistance against arresting officers.

Police records show since the crackdown started, 3,264 alleged drug offenders have been killed in gunbattles with law enforcers. More than 2,000 others died in drug-related homicides, including attacks by motorcycle-riding masked gunmen.

Human rights groups report a higher toll and demand an independent investigation into Duterte's possible role in the violence.