The current presidential frontrunner is outspoken 71-year-old mayor Rodrigo "Digong" Duterte.
But current president Benigno Aquino is leading attempts to bring together other candidates to defeat him.
The campaign has been marred by violence. On Saturday a mayoral candidate was murdered in the south.
A gunman stormed the home of Armando Ceballos, standing as an independent in the town of Lantapan, and shot him dead, police said.
It brings to 15 the number of people killed during the campaign.
Correspondents say the Philippines has a long tradition of political violence and dozens are killed during each election campaign.
On Monday voters will select a new president for a six-year term, as well as a vice-president, 12 senators and many local officials.
More than 54 million people are registered to vote across the archipelago of 7,000 islands.
Rodrigo Duterte, nicknamed "The Punisher" for his tough crime-fighting approach, has promised to eliminate crime and corruption within months of attaining office.
However, his brash speaking style and contempt for political correctness has seen him compared to US Republican frontrunner Donald Trump.
He recently joked that, as mayor, he should have been first to rape an Australian missionary murdered in a prison riot, but he later apologised.
President Aquino called on Mr Duterte's rivals to agree on a plan to narrow the field of candidates and consolidate their votes.
However, candidates including Grace Poe, Jejomar Binay and Miriam Defensor-Santiago refused to step aside.
Mr Duterte's campaign said the attempt to form an alliance against him "reeks of the stench of defeat".
The economy, crime, corruption, poverty and territorial disputes with China in the South China Sea are all major issues.
Traffic jams and Duterte fans in Manila
Filipino boxing youth and politician Pacquiao
Election issues through emojis
The fizzy drink and bun presidential poll
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Who will lead the Philippines?
Final campaign rallies are being held in the Philippines' capital Manila ahead of Monday's election.