A self-confessed hitman has testified President Rodrigo Duterte personally issued assassination orders while mayor of a city where activists say hundreds of summary executions took place.

The president made no comment on the allegations on Thursday, but his political allies dismissed them as lies.

Speaking during a senate hearing investigating the Philippine president’s anti-crime crackdown, Edgar Matobato said he heard Duterte, as mayor of Davao city in the early 1990s, give instructions to carry out extrajudicial killings.

“Our job was to kill criminals like drug-pushers, rapists, snatchers,” said the 57-year-old, adding he himself had killed more than 50 people while working for a “Davao Death Squad”.

Matobato also alleged that the president’s eldest son and Davao’s current vice-mayor, Paolo Duterte, was a drug-user who ordered the death of a hotel owner in 2014.

Rodrigo Duterte has repeatedly denied involvement in vigilantism as either mayor or president.

Rights groups have documented some 1400 suspicious killings in Davao since the early 1990s and critics say the bloody war on drugs Duterte has unleashed since taking office on June 30 bears the hallmarks of similar methods.

More than 3500 people, or about 47 per day, have been killed in the past 10 weeks, some 58 per cent by unknown assailants and the rest in legitimate police operations, according to police.

Matobato said that in the 1990s he had overheard Rodrigo Duterte order the bombing of mosques in Davao as retaliation for an attack on a cathedral.

Justice Secretary Vitaliano Aguirre described Matobato’s testimony as “lies, fabrications and a product of a fertile and a coached imagination”.

The United Nations and United States have expressed concern about his latest crackdown.

The president has told them not to interfere, using lurid language.

Little is known about Matobato, who volunteered to give testimony in a senate investigation led by Leila de Lima, a former justice minister who has denounced Duterte’s crackdown.