The protest follows the arrest of President Rodrigo Duterte's high-profile critic, Senator Leila de Lima, who was arrested on drug-related charges.
Protesters clashed with police in Manila on Saturday during a rally against Philippines' President Rodrigo Duterte's brutal war on drugs.
The demonstration comes a day after Duterte's most high-profile critic, Senator Leila de Lima, was arrested on drug-related charges.
The rally began peacefully, but later turned violent when a clash erupted between a group of people and police.
Law enforcement authorities resorted to using a water canon to disperse the crowd which injured some people.
Earlier, the crowd gathered outside the national police headquarters where de Lima was detained on Friday, to protest against the alleged death-squad murders of thousands of drug suspects.
More than 7,000 drug suspects have been killed by police and unknown assassins since Duterte assumed power in July last year.
Protesters warned that Duterte's crackdown could lead to a repeat of the military-backed dictatorship of Ferdinand Marcos, which was toppled in a bloodless revolution 31 years ago.
Bonifacio Ilagan, who led one of the protests outside the police headquarters said, "We are taking the matter seriously. We are warning our people about the threat of rising fascism."
More protests were expected later with thousands of De Lima supporters of set to join another rally.
De Lima, a former human rights commissioner, said she was being persecuted for her decade-long efforts to expose Duterte as the leader of death squads when he was a mayor of Davao.
An overnight vigil to demonstrate public backing for Duterte's drug crackdown was also scheduled in Manila later Saturday.
TRT World 's Sarah Jones reports.