Millions of Filipinos braved long lines and soaring temperatures Monday to vote in a mid-term election largely defined by the explosive feud between President Ferdinand Marcos and impeached Vice President Sara Duterte.

With temperatures hitting 34 degrees Celsius (93 Fahrenheit) in some places, George Garcia, head of the Commission on Elections (Comelec), said some voting machines were "overheating".

"It's slowing the voting process," he told reporters at a prison in southern Manila where inmates were casting ballots.

"Due to the extreme heat, the ink (on the ballots) does not dry immediately, and the ballot ends up stuck on the scanners," Garcia said, adding officials in some areas were resorting to aiming electric fans at the machines.

Monday's election 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.

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