More than 100,000 police officers are on duty amid violence ahead of voting.
Polls opened across the country at 6am local time on Monday (22:00 GMT Sunday) and the election pits frontrunner Rodrigo Duterte, an outspoken mayor from the city of Davao, against four other contenders.
The campaign has been driven by public concerns about the economy, inequality and rampant corruption.
At least seven people were killed at the same time in an ambush by unknown gunmen in the town of Rosario, in Cavite province, south of the capital, Manila ahead of elections.
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