MANILA (Reuters) - A Philippine mayor who paraded suspected drug dealers through the streets was shot dead on Monday while attending a weekly flag-raising ceremony, police said.

Mayor Antonio Cando Halili was hit by a single bullet to the chest as he and civil servants sang the national anthem in Tanauan, a city in Batangas province southwest of the capital, Manila.

“We are shocked, we are saddened,” Vice Mayor Jhoanna Villamor, who was standing beside Halili, told radio station DZBB.

What appeared to be a smartphone video of the shooting went viral on social media, showing a single shot ringing out as the anthem played, then screaming and pandemonium. The video could not be immediately verified.

Halili gained prominence for introducing a “walk of shame” parade of alleged drug dealers through Tanauan.

Police say they have killed more than 4,200 suspected drug dealers during shootouts in a bloody war on drugs launched by President Rodrigo Duterte two years ago, a campaign condemned by domestic and international human rights groups.

Halili was stripped of his supervisory powers over local police in October 2017 due to a proliferation of illegal drugs in his city, amid allegations by the national police that he may have been involved. Halili denied the allegations.

In an interview with Reuters in August 2016 - the second month of the crackdown - he said he backed Duterte’s campaign but believed drug kingpins should be the main targets, otherwise thousands of people would be killed.

He expressed concern over the way police conducted the war on drugs and the reliability of their intelligence, and that he might be accused of colluding with narcotics gangs.

“No one is safe – mayors, governors, congressmen – just a false intelligence report by the police can end up with any of them being destroyed,” he said in the interview.

“I have a feeling they (police) are going after the small fry to frighten the people,” he said.

Police are investigating Monday’s shooting. One investigator told a radio station that a high-powered rifle was used in the attack.

Duterte’s spokesman, Harry Roque, vowed justice for the mayor, describing him as a “staunch advocate against illegal drugs”.

Duterte, speaking at an airport inauguration in the central Philippines, said merely that he suspected drugs were behind the killing. He described Halili as “the one who pretended to parade the addicts”. He did not elaborate.