In a last-ditch effort to save democracy and stop a looming "dictator" and "monster" from taking over the Philippines, President Benigno Aquino III on Friday called on two rival candidates to unite to defeat Rodrigo Duterte, who continues to hold a double-digit lead in the polls ahead of Monday's presidential election.

Aquino said he saw encouraging signs that a last-minute coalition between presidential candidates Mar Roxas and Grace Poe would materialise to defeat Duterte, a town mayor who continues to enjoy the support of the masses despite accusations of corruption and hidden wealth, the Philippine Daily Inquirer reports.

The final survey taken before Monday's election showed Duterte getting the support of 33 percent of Filipino voters. Poe is at second with 22 percent while Roxas is at third with 20 percent.

Despite the dim forecast, Aquino said he remains hopeful that the anti-Duterte forces would prevail in the end. "There are both known and unknown efforts [toward a coalition] these days when our democracy is threatened," he told Inquirer editors and reporters in an interview in Malacanang.

The only son of Philippine democracy icons Ninoy and Cory Aquino said contrary to the poll surveys, he believes majority of the Filipinos are against a Duterte presidency.

Aquino said if Roxas and Poe would decide to unite, that would be more than 40 percent voter support to defeat Duterte's 33 percent.

Aquino admitted that Duterte's rising popularity has been a cause for concern because of the likelihood that the Philippines would end up wasting its hard-fought freedom from two decades of martial rule under dictator Ferdinand Marcos.

The President said he could not understand why many Filipinos believe in the rosy promises of good government by a politician who has admitted participation in thousands of extrajudicial killings and being a womaniser among other outrageous charges.

"The closest analysis [of his promises] is 'I will give the moon and the stars and all of that.' We know that cannot be done but it seems acceptable [to many people]," Aquino said.

The President said he is particularly worried by Duterte's threat to put up a revolutionary government, kill crime suspects outside the judicial system, and sever ties with Philippine allies the United States and Australia.

"If he starts doing all of these, then really there is a threat to [Philippine democracy]," he said.

Former Philippines Justice Secretary Leila de Lima recently called Duterte "a monster, who has to be stopped by all means."

Earlier, the archbishop of Mindanao, where Duterte is based, issued an extraordinary pastoral letter linking Duterte to more than 1,400 extrajudicial killings in Davao City where he is the mayor.

Cagayan de Oro Archbishop Antonio Ledesma also condemned Duterte for his adulterous affairs, calling them "sinful and immoral," Rappler reports.

Catholic bishops do not explicitly campaign for or against any specific candidate, making Ledesma's direct attack on Duterte remarkable.

Ledesma said Duterte should be held accountable for the extrajudicial killings in Davao City. The archbishop cited a "deeply disturbing report" from the Commission on Human Rights and Human Rights Watch that from 1998 to 2015, "1,424 extrajudicial killings" were recorded in Davao City. Among those slain were 132 children, he added.

Duterte himself once told an editor of Rappler: "When I said I'll stop criminality, I'll stop criminality. And if I have to kill you, I'll kill you. Personally."

The Catholic Bishops' Conference of the Philippines (CBCP) earlier blasted Duterte for cursing Pope Francis, calling the head of the Roman Catholic Church "the son of a whore."

Duterte also drew rebuke when he commented during recent election campaign rally on the 1989 gang rape and murder of Australian nun Jacqueline Hamill during a prison siege in Davao. Duterte reportedly said: "She was so beautiful. I thought the mayor should have been first."

The comment triggered a wave of criticism both locally and internationally, particularly in Australia.