MANILA — A mayor in the Philippines known for parading drug suspects through his town was fatally shot during a flag-raising ceremony on Monday morning, the police said.

Antonio Halili, the mayor of Tanauan City, about 40 miles south of Manila, was shot in the chest by a gunman and died immediately, the police said. The gunman remained at large.

“Mayor Halili is a staunch ally of the president in the war on drugs,” Harry Roque, a spokesman for President Rodrigo Duterte, said in a brief statement. “He was a great mayor. This is such a loss.”

Mr. Halili was the fourth mayor killed during Mr. Duterte’s war on drugs, which has drawn fierce condemnation from rights groups for extrajudicial killings of thousands of drug suspects.