The event dubbed "Walk for Life" gathered priests, nuns and students criticizing government's policies, including the legislation allowing divorces and re-imposing capital punishment in the country, the CNN Philippines reported.
The demonstrators were carrying banners and placards, saying "Stop the Killings," "Protect&Defend the Sanctity of Life&Marriage," as well as "End Impunity."
READ MORE: Philippine Ruling Bloc's Senators Call for Probe Into Killings in 'War on Drugs'
The large-scale anti-drug campaign, known as the "war on drugs," was launched by Duterte in June 2016 following his election promises to end drug-related crimes in the country.
Since then, thousands of suspected drug users and dealers have been killed by Philippine National Police officers or unknown gunmen. The campaign has been sharply criticized by the international community and human rights groups.