A supporter poses beside a life-size cutout of leading presidential candidate Mayor Rodrigo Duterte at his hometown in Davao city in southern Philippines Wednesday, May 11, 2016. Duterte has widened his lead in unofficial tally but still refuses to claim victory.(AP Photo/Bullit Marquez)

MANILA, Philippines - Philippine election officials challenged Sen. Ferdinand "Bongbong" Marcos Jr. on Wednesday to prove his allegation of irregularities in the counting of votes for vice-president, where he has been overtaken by his closest rival.

They also rejected Marcos' request for a stop to the unofficial tally by an accredited citizens' watchdog, which uses the same election returns that are transmitted to the Commission on Elections.

The son of late dictator Ferdinand Marcos Sr. had initially led the partial count by the watchdog known by its acronym PPCRV. But as of Wednesday afternoon, the administration's candidate, Rep. Leni Robredo, was leading by more than 230,000 votes, putting her 0.6 percentage points ahead of Marcos.

The tally is based on results from 95.5 per cent of precincts nationwide. Ballots from overseas Filipinos are now considered crucial in the race for vice-president.

Rodrigo Duterte, the bombastic mayor of southern Davao city, is poised to become the new president, based on PPCRV results that gave him an unassailable lead.

The official count and proclamation of the president and vice-president are done by Congress, which will convene May 24.

If Marcos wins, that would put him a step away from the presidency 30 years after his late father was ousted by a public uprising amid allegations of plunder and widespread human rights abuses.

On Tuesday, Marcos campaign adviser Rep. Jonathan dela Cruz said they sent an urgent request to the Commission on Elections to halt the PPCRV count because it showed "an alarming and suspicious trend" contrary to independent exit polls and the campaign's estimates.

"These accusations are not true ... we are committed to being impartial, to be neutral," elections commission chairman Andres Bautista said.

He said any complaint would be acted upon based on evidence.

Election Commissioner Rowena Guanzon said there was no reason to stop the official count.

Marcos appealed to his supporters, who have been calling through social media for protest rallies, to stay calm.

The Marcos family fled to Hawaii four days after a 1986 "people power" uprising in which rosary-clutching nuns and ordinary citizens knelt before tanks, and protesters stuck yellow flowers into the muzzles of assault rifles of pro-government troops. His father died in exile three years later, after denying any wrongdoing.

After the Marcos family returned to the Philippines in 1991, Marcos Jr. became governor, congressman and, in 2010, a senator.

President Benigno Aquino III, whose parents were democracy champions who helped topple the senior Marcos, campaigned against his son, who has never clearly apologized for abuses of his father.

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This story has been corrected to show that the tally is based on results from 95.5 per cent of precincts, not votes.

News from © The Associated Press, 2016