Thousands of Roman Catholics marched in the Philippines capital Manila on Saturday in the biggest gathering denouncing extra-judicial killings and a government plan to reimpose the death penalty for criminals.

Dubbed a "Walk for Life" prayer rally and endorsed by the Catholic Bishops Conference of the Philippines (CBCP), the gathering came just days after the church launched its strongest attack against President Rodrigo Duterte's war on drugs.

Organisers claimed as many as 50,000 people took part in the march toward Manila's Rizal Park, while about 10,000 based on police estimates stayed to hear speeches.

More than 7,600 people have been killed since Duterte launched his anti-drugs campaign seven months ago. More than 2,500 died in shootouts during raids and sting operations, according to the police.