Millions of Filipinos began casting their votes in polling centers across the country on Monday to pick a new president.

Five candidates are vying for the presidency to succeed Benigno Aquino III, whose six-year term ends on June 30.

Rodrigo Duterte, mayor of the south Philippine city of Davao, has been leading most of the opinion polls since April.

Some 33 percent of the voters said they would vote for Duterte in the final pre-election survey conducted by Social Weather Station. The country's renowned polling body carried out the survey from May 1 to 3 through face-to-face interviews with 4,500 validated voters nationwide with a sampling error margin of 1 point.

The 71-year-old mayor is known to have made Davao one of the safest cities in the Philippines through tough regulations and iron-fist approach in crime fighting.

During the presidential campaign period, he vowed to wipe out corruption, drugs and criminality in three to six months if he wins the election. In his "miting de avance," a gathering of supporters of respective candidates where they would make their last appeal of support, held on Saturday, he called out to criminals to assassinate him on the night. He said otherwise he would kill them all when he became the president.

His aggressive behavior and offensive language won him a lot of supporters, but aroused concerns and controversy as well.

The outgoing president Aquino has warned repeatedly the risk for the country to fall under a dictatorship if citizens voted Duterte.

Analysts warned possible political instability if Duterte became the president as his "revolutionary government" would be most unlikely to get support from the congress.

Senator Grace Poe, 47, had 22 percent of support in Social Weather Station's latest survey, being the second most popular candidate.

Adopted child of the late movie actor Fernando Poe Jr. and a first-term senator, Poe's fresh-face and her care for the poor has won over many voters in the early stage of the campaign. But her previous U.S. citizenship and the tortuous process to prove herself a natural born Filipino to be qualified for election hurt her.

Liberal Party standard bearer Manuel A. Roxas II's supporting rate remains at 20 percent in the latest survey. He is the grandson of the former Philippine President Manuel Roxas. He served in the cabinet of the Philippines as Secretary of the Interior and Local Government from 2012 to 2015. He is supported by the outgoing president Aquino. But he left some people the impression of indecisive and lacking charisma.

The incumbent vice president Jejomar Binay was the first among five to announce his running for presidential election. He was once believed to be the most competitive candidate for this election but supports slipped away after he was caught in a corruption scandal. He only got 13 percent of support in the survey.

Senator Senator Miriam Defensor-Santiago got 2 percent of support. The survey said 4 percent were undecided.

Besides electing a new president, voters will also be choosing candidates for vice president, hundreds of lawmakers and around 18,000 local officials.

Some 54 million Filipinos are eligible to vote in Monday's polls, which opened at 6:00 a.m. local time (2200 GMT Sunday) and will close at 5:00 p.m. local time. The election commission said unofficial outcome for the presidential and vice presidential polls will be expected within 72 hours.

Roxas has already voted in Roxas city at 8 a.m. this morning, Binay voted in Makati city, Poe in San Juan, Santiago in Quezon city and Duterte will vote in Davao city at 2 p.m..

Voting was delayed in some parts of the country early this morning due to vote counting machine malfunctioning such as rejected ballots and paper jams.