He said the poll body expected a high turnout among the 67.5 million registered voters, of whom more than seven million would cast their ballots for the first time.

At stake are 18,180 political positions, including a six-year-term president, 12 members of the Senate who will also serve for six years, and thousands of governors, mayors and members of local legislative councils who will serve for three.

While six pre-election surveys have singled out former senator Ferdinand “Bongbong” Marcos Jnr as the runaway winner, two new surveys, one using a different methodology and disclosed on Friday have predicted a “dead heat”, come-from-behind win for his main rival Leni Robredo.

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As Robredo voted in her home province of Camarines Sur on Monday, she said there was concern over reports of election-linked violence and malfunctioning vote-counting machines. She called on the election commission to show that it was on top of the situation, adding that vote integrity was at stake.

Academics led by sociologist and statistician Dr Imelda Pagtolun-an, former dean of Xavier University’s College of Arts and Sciences, said they had asked over a thousand volunteers to carry out face-to-face interviews with 4,800 registered voters across the country from April 18 to 22, and found 57 per cent support for Robredo and 38 per cent for Marcos. The results were the opposite of six other surveys showing Marcos the runaway winner.

Separately, in a Zoom forum on May 6, the Centre for People Empowerment in Governance (CenPEG) think tank and Shofar Analytics Group presented results of its survey using around 40,000 text messages sent daily to randomly-computer-generated local mobile phone numbers from March 11 up to May 5.

02:37

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Dormant nuclear plant in Philippines could be revived if ‘Bongbong’ Marcos wins presidency

The replies were then validated with non-registered voters weeded out and they received around 200 “valid responses from unique numbers per day”, said Dr Felix Muga II, an associate professor in the mathematics and engineering department of Ateneo de Manila University.

Muga explained that the groups’ method was similar to that used by the Trafalgar Group in the United States, which was the lone polling firm to correctly predict a Donald Trump win in the 2016 presidential election.

Muga said their results did not contradict the findings of survey firms Pulse Asia and Octa, which had both released figures showing Marcos Jnr was way ahead. But he added that while Pulse Asia’s latest survey finished on April 21 and Octa’s on April 25, they continued to do daily polls using this method and they ended Friday midnight.

“Our data [so far] shows it’s a dead heat,” said Sid Balatan, who also took part in the project. On Saturday, Muga issued a press statement saying that based on their results as of Friday midnight, Shofar Analytics predicted a “Leni win” by a 2.5 per cent margin “with 85.5 per cent probability”.

Muga pointed out that, like the Trafalgar poll, their survey – because it was anonymously done instead of face-to-face interviews – removed “the fear factor” of the respondents. Balatan noted that factor was present especially during the Duterte administration, where extrajudicial killings had been rampant and respondents might be afraid to answer truthfully for fear of retaliation or not receiving ayuda, or pandemic aid.

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In a separate Zoom forum on the election, political scientist Jean Franco from the University of the Philippines noted that respondents in face-to-face interviews may not necessarily be telling the truth, but instead said what the interviewer or those around them wanted to hear.

She noted that an exit poll of voters conducted during the May 2016 elections had contradicted the poll firms’ pre-election day findings that only two per cent of voters were “undecided”.

In fact, Franco said, the 2016 exit poll of Social Weather Stations showed that 18 per cent of voter-respondents said they decided on their presidential candidate only on election day, and another 15 per cent of voter-respondents said they decided only in the previous week.

One factor that could have sealed Marcos Jnr’s win would have been the personal endorsement of President Duterte.

But on May 6, Duterte publicly said: “I’ll say this again, OK? I’m not supporting any presidential candidate. It is up to you who you want to vote. I won’t dictate who you’ll vote. I’m neutral.”

He is, however, backing his daughter Davao City mayor Sara for the post of vice-president.

Earlier, Duterte had described Marcos Jnr as “spoiled” and “weak”, and insinuated that he was a drug addict.

Businessman Rafael Ongpin said it was “very hard to tell at this point (who would win). It’s not clear. The surveys say Marcos, the crowd says Leni”.

03:51

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‘It makes my blood boil’: torture victims denounce Marcos family before Philippines election

On Saturday, both candidates held their final rallies to what each side claimed were record crowds of over one million people. It was not clear, though, how the Marcos camp could have transported that many people, since each bus could only contain 100 which would mean fielding 10,000 buses – or more than double the number plying Metropolitan Manila.

Later, organisers for Robredo adjusted their estimate to 750,000 people attending the rally in the heart of the country’s financial district of Makati City. Catriona Gray, the 2018 Miss Universe, made a surprise appearance.

“We need to remember that the government has the power to change our lives but we have the power to change the government,” Gray said of Robredo.

Marcos Jnr, meanwhile, staged his final rally in a huge vacant lot inside Entertainment City, Metro Manila’s gambling complex which was far from any public transport route. He again spoke about unity and the need to guard the votes against any cheating, since he claimed he was cheated by Robredo in 2016.

However, it was the Supreme Court which ruled by a unanimous vote that Robredo clearly won in 2016 by a small margin.

Additional reporting by Bloomberg