The shooting happened shortly after voting got under way in Buluan municipality on Mindanao island.
At least three members of local peacekeeping forces have been shot dead by assailants near a polling station in the southern Philippines during national elections.
The deadly shooting happened shortly after voting started in Buluan municipality on Mindanao island on Monday. Maguindanao provincial police spokesman Major Roldan Kuntong said a fourth guard was wounded in the attack.
Former Mayor Ibrahim Mangudadatu said people inside the school being used as a polling station ran for cover when the shooting started.
Armed men also attacked a polling station in Lanao del Sur province in Mindanao, killing one voter and wounding two others, police said.
Police said the victims walked from their remote mountain villages to cast their votes at the municipal hall when polling stations opened across the country on Monday morning.
“It is their custom to come down early from their villages, which are located eight to 12 hours away on foot,” said Kuntong.
A spokesman for the Commission on Elections said they were trying to verify if the attacks were election-related.
Presidential candidate Leni Robredo, who voted in her home province of Camarines Sur, expressed her worries after hearing of the incident.
“I hope the authorities will show they are on top of everything,” she told reporters.
Five grenades exploded outside a polling station in Datu Unsay municipality in Maguindanao province, injuring nine people on Sunday.
Thousands of police personnel and armed forces have been deployed across the archipelago to help secure polling stations and ballots and guard checkpoints and escort politicians.
In addition to the presidential election, thousands of posts including the vice presidency, Senate and House of Representatives seats are also being contested.
In the 2016 presidential elections, 133 violent incidents were recorded.