A cellphone video shot by Alcazar shows a few men standing around the fallen Halili as gunfire rang out continuously and people cried, screamed, ran and took cover during the melee. A man yelled, 'The mayor is dead, the mayor was shot,' and another desperately called for a car to take Halili to the hospital. A third man started blaming his companions for the security breach.

"They did not see anybody approach him. They just heard a gunshot so the assumption or allegation was it could have been a sniper shot," Albayalde said in a news conference in Manila, adding that an investigation was underway.

The bullet hit a cellphone in Halili's coat pocket then pierced his chest, police said. Policemen scoured the hill but failed to find the gunman.

Halili two years ago ordered drug suspects to be paraded in public in Tanauan, a small city about 70 kilometres south of Manila, in a campaign that was dubbed "walks of shame". The suspects were forced to wear cardboard signs that read "I'm a pusher, don't emulate me" in a campaign that alarmed human rights officials.

Police officials, however, also linked Halili to illegal drugs, an allegation he strongly denied. He said at the time that he would resign and would be willing to be publicly paraded as a drug suspect if police could come up with evidence to support the allegation.

Albayalde said investigators would try to determine if the killing was connected to Halili's anti-drug campaign.

Halili's unusual campaign drew attention at a time of growing alarm over the rising number of killings of drug suspects under President Rodrigo Duterte. Since Duterte took office in 2016, more than 4200 drug suspects had been killed in clashes with police, alarming human rights groups, Western governments and UN rights watchdogs.

Human rights groups have reported much higher death tolls, although Duterte and his officials have questioned the accuracy of those reports. They said the suspects died because they opened fire and sparked gunbattles with authorities although human rights groups have accused police of extrajudicial killings.

Halili's killing came a few weeks after a Catholic priest was shot and killed while preparing to celebrate Mass at the altar of a village chapel in northern Nueva Ecija province.

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Senator Panfilo Lacson, a former national police chief, urged the police to impose stricter firearms control in light of the killings.

"The killing of priests, prosecutors, and former and incumbent local officials in broad daylight and in full view of the public may be suggestive of the impunity and brazenness of those responsible for such acts," Lacson said.

"The Philippine National Police should feel challenged, if not taunted," he said. "And they must immediately consider stricter firearms control strategies before similar killings could reach ubiquitous levels."

AP