Polls were officially open at 6am local time (22:00 GMT on Sunday) and will close at 6pm (10:00 GMT).

More than 60 million Filipinos are eligible and registered to vote in the midterm polls, with roughly 43,000 candidates going for some 18,000 government posts.

Voter surveys for most preferred candidate by private pollsters predict a favourable result for the administration, with its senatorial candidates poised to win up to two-thirds of the contested seats.

Critics have so far expressed fears that a win for Duterte’s allies could reduce the Senate’s independence and prevent it from keeping a check on the president, whom they expect to further push for his platforms as his single six-year term enters its home run.

“Clearly, there are few who make a stand in the government nowadays,” said Senator Leila De Lima, jailed for illegal drug charges after she ran an investigation on thousands of killings in Duterte’s “war on drugs”.

“Our institutions lack voices for justice and truth. Many fear persecution and choose to kowtow just to stay in power,” she said in a statement on Monday.

One out of four incumbent opposition senators, De Lima urged voters to “reject the liars, the corrupt, the plunderers!”

This was a clear jab at Duterte’s senatorial slate, which includes two former senators charged with plunder and a daughter of the late dictator Ferdinand Marcos.