MANILA (AFP) - Philippine presidential favourite Rodrigo Duterte joked on Wednesday (April 27) about his penis and vowed to pardon himself for mass murder, as he took his profanity-laced campaign to the nation's business elite.

Mr Duterte also hit out again at key Philippine allies the United States and Australia, after warning last week he would be prepared to sever diplomatic ties because their ambassadors spoke out against a joke he made about raping a missionary who was later murdered.

Mr Duterte has shocked the political establishment in recent weeks - surging to a clear lead in opinion surveys before the May 9 presidential election with a campaign stump speech full of swear words and promises to end crime by killing tens of thousands of criminals.

In an address to the nation's premier group of business figures on Wednesday, Mr Duterte defended his history of womanising with typical straight-talking and crassness.

"I was separated from my wife. I'm not impotent. What am I supposed to do? Let this hang forever," Mr Duterte said, referring to his penis, as people in the crowd laughed. "When I take Viagra, it stands up."

Mr Duterte, 71, had earlier on the campaign trail said he had two mistresses, but reassured taxpayers they would not cost much, because he kept them at cheap boarding houses and took them to short-time hotels for sex.

Mr Duterte has also boasted about running vigilante death squads during his many years as mayor of Davao, the biggest city in the southern Philippines.

He once said the death squads had killed 1,700 people. However he has at other times denied any links to the vigilante groups, which have attracted fierce criticism from human rights groups and the United Nations.

A key part of Mr Duterte's campaign has been a pledge to eradicate crime nationwide in the first six months of his presidency by ordering security forces to kill criminals.

In his speech to the Makati Business Club, Mr Duterte again promised that his presidency "will be a bloody one".

He promised to pardon soldiers or police officers if they were charged with rights abuses.

"I will issue 1,000 pardons a day," Mr Duterte said.

Mr Duterte said the Constitution also allowed the president to pardon himself for crimes, and he would do so at the end of his term.

"Pardon given to Rodrigo Duterte for the crime of multiple murder, signed Rodrigo Duterte," he said, as people in the crowed laughed.

Surveys have shown that Mr Duterte is particularly popular among the nation's wealthy and middle class, who fear rising crime.

Mr Duterte last week joked that he had wanted to rape a "beautiful" Australian missionary, who was sexually assaulted and murdered by inmates in a 1989 prison riot in Davao.

"I was mad she (the missionary) was raped but she was so beautiful. I thought, the mayor should have been first," he said at a rally.

The Australian and US ambassadors criticised his comments, prompting Mr Duterte to tell them to "shut their mouths" and caution that diplomatic ties were at risk if he became president.

On Wednesday, he criticised the United States and Australia for failing to do more as China expanded its presence in areas of the South China Sea that are also claimed by the Philippines.

"Why the hell didn't Australia, the Americans or even Japan stop it?" Mr Duterte said.

In the Philippines, presidents can serve only a single six-year term.

Mr Duterte has tapped into strong sentiments among many Filipinos who are frustrated with the slow pace of reforms under current President Benigno Aquino, according to analysts.

They say many voters are being lured by his hard-man image and promises of quick fixes to deep-rooted problems such as crime and corruption.