MANILA — One of the founding members of a Philippine lawyers’ group at the forefront of opposing President Rodrigo Duterte’s lethal war on drugs was gunned down on Tuesday, killed by three bullets as he was leaving his office for the night.

The attorney, Benjamin Ramos, 56, was the 34th lawyer killed since Mr. Duterte became president two years ago. His group, the National Union of People’s Lawyers, specialized in doing no-cost work for poor clients whose families have been targeted by the police, soldiers and death squads associated with the president’s drug war.

“We are shocked, devastated and enraged at the premeditated, coldblooded murder of our colleague and fellow people’s lawyer,” said Edre Olalia, a leader of the group. “We are disturbed and unbowed. These are dangerous times.”

Initial police reports said that Mr. Ramos had just finished work when he was shot by motorcycle-riding men near the central town of Kabankalan on Tuesday night. He was declared dead on arrival at the hospital.

Colleagues described Mr. Ramos as passionately dedicated to pro-bono work for the poor, environmentalists, activists and political prisoners.

The National Union of People’s Lawyers said his work had angered local policemen and the military. His picture was recently included by the local police in a list of people accused of having ties to the underground communist movement, a claim that the group has denied.

Jose Manuel Diokno, a human rights lawyer for another organization, the Free Legal Assistance Group, said he was outraged by the killing.

“I join the calls for the police to conduct an impartial and thorough investigation, to find the motive, the gunmen and the mastermind,” Mr. Diokno said in an interview, urging the Justice Department to take the lead in the inquiry.

“I ask my fellow lawyers who fight for what is right and just to continue and not be cowed,” he said.

In a speech in August 2017, Mr. Duterte told the national police not to be daunted by rights lawyers investigating the thousands of deaths of what he called drug addicts and dealers.

“If they are obstructing justice, you shoot them,” Mr. Duterte instructed the police, referring to lawyers. (The president is himself a lawyer and began his political career as a prosecutor.)

He often carries a list of the names of dozens of judges, members of the armed and police forces as well as of local politicians he claims are protecting drug suspects.

He has never publicly divulged how he came up with the list, although at least three mayors whose names appeared on it have been killed. Eight other mayors were also gunned down in suspicious circumstances. One of them was killed last year in a raid that left his wife and 10 other people dead. A mayor was also fatally shot during a flag-raising ceremony in July.

Mr. Olalia said that some of the National Union of People’s Lawyers members had received threats, and had been labeled by the police and the military as sympathetic to the political left, in “open contempt of basic principles of the role of lawyers in democracy.”

He said a colleague investigating abuses against women and children, Katherine Panguban, had recently been criticized by the police for leading a fact-finding mission into the killings of nine farmers in the central city of Sagay.

The military has said the farmers were recruited by a front organization for communist rebels, and attributed their deaths to infighting among the cadres.

Mr. Olalia said Mr. Ramos’s death would not stop the group’s work.

“We will be there in the trenches in defense of the defenseless,” he said. “There is no other choice.”

The group Human Rights Watch said Mr. Ramos’s killing was proof that impunity existed under Mr. Duterte’s drug war.

“It is a blow to the human rights movement in the Philippines,” the group said in a statement. “We demand an impartial investigation into Ramos’s murder and the many other attacks against lawyers in the Philippines and that the authorities bring the perpetrators to justice.”