Millions of Filipinos headed to the polls Monday in a mid-term election widely seen as a referendum on the explosive feud between President Ferdinand Marcos Jr. and impeached Vice President Sara Duterte.

Voters braved long lines and temperatures that reached 33 degrees Celsius (91 Fahrenheit) as it approached midday in Manila, polls having officially opened at 7:00 am.

The race will decide more than 18,000 posts, from seats in the House of Representatives to hotly contested municipal offices. 

It is the battle for the Senate, however, that carries potentially major implications for the presidential election in 2028.

The 12 senators chosen Monday will form half the jury in a Duterte impeachment trial later this year that could see her permanently barred from public office. 

Duterte's long-simmering feud with former ally Marcos erupted in February when she was impeached by the House for alleged "high crimes" including corruption and an assassination plot against the president.

Barely a month later, her father, former president Rodrigo Duterte, was arrested and sent to the International Criminal Court to face a charge of crimes against humanity over his deadly anti-drugs campaign.

On Monday, 53-year-old Roland Agasa, one of the country's 68 million registered voters, said the feud had taken a mental toll heading into election day.

"VP Sara and Bongbong (Marcos), for me, the government is getting stressful," he said outside a Manila elementary school hosting a polling station on its fourth and fifth floors.

"I hope we choose the deserving, those who can help the country," Agasa said, adding he planned to wait until the day cooled before braving the stairs to cast his vote.

"There was no pushing, but it was cramped. It was difficult, but we endured so that we could vote," Rizza Bacolod, 32, said at the same location.

"I still believe that change can happen (in the Philippines)."

President Marcos cast his vote at an elementary school in his family's traditional stronghold of Ilocos Norte province. His mother Imelda, 95, was at his side.

Numbers game

Sara Duterte will need nine votes in the 24-seat Senate to preserve any hope of a future presidential run. 

Heading into Monday, seven of the candidates polling in the top 12 were endorsed by Marcos while four were aligned with his vice president.

Two, including the president's independent-minded sister Imee Marcos, were "adopted" as honorary members of the Duterte family's PDP-Laban party on Saturday.

The move to add Marcos and television personality Camille Villar to the party's slate was intended to add "more allies to protect the Vice President against impeachment", according to the resolution.

Despite his detention at The Hague, Rodrigo Duterte remains on the ballot in his family's southern stronghold of Davao city, where he is seeking to retake his former job as mayor. 

One local poll is predicting he will win comfortably.

Election violence

A day before the election at least two people were killed in a clash between supporters of rival political camps in southern Mindanao island's autonomous Muslim region, the Philippine army reported.  

An official who answered the phone at the Basilan province disaster office in the Bangsamoro Autonomous Region put the death toll at four.

The Philippines has a history of violent elections, especially in the restive south, where armed clashes between groups of political rivals are common.

National police have been on alert for more than a week and around 163,000 officers have been deployed to secure polling stations, escort election officials and guard checkpoints.

Thousands more personnel from the military, fire departments and other agencies have been mobilized to keep the peace in a country where battles over hotly contested provincial posts are known to erupt in violence.

A city council hopeful, a polling officer and a village chief are among the at least 16 people police say have been killed in attacks in the run-up to Monday's election.

A special early voting window this year allowed the elderly and people with disabilities to begin casting votes at 5:00 am.