Thousands of protesters marked the anniversary of the 1972 martial law declaration by late Philippine dictator Ferdinand Marcos with an outcry against what they say is the current president's authoritarian tendencies and his brutal crackdown on illegal drugs.
Hundreds of riot police deployed to secure the marches and rallies, among the largest against President Rodrigo Duterte since he rose to power last year.
Waving red flags and carrying placards that read "Stop the Killings," left-wing groups started to gather in three Manila areas, including at the foot of a bridge leading to the presidential palace.
Duterte has warned he would use force or expand nationwide his martial law declaration in the country's south if anti-government protesters threaten public order.
He placed the entire southern third of the largely Roman Catholic nation under martial law in May to deal with a bloody siege by pro-Islamic State group militants in southern Marawi city that has dragged on for nearly four months.
Known for bombastic remarks, Duterte has said activists can stage street protests without government permits, and that even communist guerrillas can join as long as they don't bring their guns.
He said he was also ready to turn in his resignation to Congress anytime if the military agrees.
The unorthodox president has remained popular in his crime-weary country, helped by his common-guy demeanor and populist rhetoric.
Duterte said on Wednesday he has ordered law enforcers to kill his own children, two of whom serve as mayor and vice mayor of his southern home city, if they're found to be involved in illegal drugs.