The crime-fighting Philippine mayor who has emerged as surprise frontrunner in presidential elections has said that he would kill his own children if he discovered they were taking illegal drugs.

Even before the last presidential debate, there was little doubt about the strongman reputation of Rodrigo Duterte, who has promised mass killing of suspected criminals after supporting vigilante gangs in his home city of Davao.

And the mayor, who proudly goes by the nicknames “The Punisher” and “Duterte Harry”, burnished those credentials when he vowed to kill his offspring if they used narcotics.

Mr Rodrigo, 71 took a commanding lead in the latest opinion polls on Monday, despite facing a backlash for making a joke about raping a murdered Australian missionary.

In the first survey taken since his latest controversial remark, his support surged six points to 33 per cent, giving him a nine per cent lead over Senator Grace Poe, his closest contender in the five-candidate field.