A Philippine presidential candidate whose comments about rape provoked an international outcry has emerged as the "clear frontrunner" in the latest opinion survey, pollsters said Monday.

Rodrigo Duterte sparked revulsion among women's groups, diplomats and the Catholic church when he told an audience that as the local mayor, he should have been at the front of the queue when an Australian missionary was attacked in a 1989 jail riot.

But far from damaging his prospects in next month's presidential vote, the remarks appear to have had little effect, with the latest opinion poll giving him a nine-point lead over his nearest rival.

Analysts say Duterte's profanity-laced campaign resonates in a chaotic, high-crime society with limited opportunities for a vast underclass working for a tiny elite. Duterte, the tough-talking mayor of the southern city of Davao, is the "clear frontrunner," research institute Social Weather Stations (SWS) said Monday.

"Mayor Duterte has been steadily gaining ground. It's a clear lead. The joke could have affected him in such a way that his score could have even been higher had it not been for that news," SWS spokesperson Leo Laroza told AFP.

More than 50 million people in the mainly Catholic nation are qualified to vote on May 9, when Duterte will face off against four other candidates, including the preferred successor of popular outgoing president Benigno Aquino.

Duterte is hailed locally as "Dirty Harry" and has boasted of operating death squads in Davao targeting criminals - a policy he has promised to introduce nationwide if elected.

In a debate Sunday, he even vowed to kill his own children if they ever took drugs.

The survey of 1,800 people, which was carried out in the days after Duterte's rape comments gained widespread publicity, showed his support rate had risen to 33 percent in April, up from 27 percent in March.