Filipinos voted on Monday in midterm elections highlighted by a showdown between President Rodrigo Duterte's allies who aim to dominate the Senate and opposition candidates fighting for checks and balances under a leader they regard as a looming dictator.

Nearly 62 million Filipinos have registered to choose among 43,500 candidates vying for about 18,000 congressional and local posts, including 81 governors, 1,634 mayors and more than 13,000 city and town councillors in 81 provinces, in one of Asia's most rambunctious democracies.

Many see the elections as a crucial referendum on Duterte’s rise to power with a brutal crackdown on illegal drugs that has left thousands dead, his unorthodox leadership style, combative and sexist joke-laden outbursts and contentious embrace of China.

“President Duterte’s name is not on the ballot but this is very much a referendum on his three years of very disruptive — yet very popular — presidency,” Manila-based analyst Richard Heydarian said.

The outcome of the polls would show whether the Filipino populace affirms or rejects Duterte’s authoritarian-style leadership in an Asian bastion of democracy, Heydarian said.

The most crucial race is for 12 seats in the 24-member Senate, which Duterte wants to fill with allies to bolster his legislative agenda. That includes the return of the death penalty, lowering the age for criminal liability of child offenders and revising the country’s 1987 constitution primarily to allow a shift to a federal form of government, a proposal some critics fear may be a cover to remove term limits.

Military and police forces were on full alert to respond to any violence, especially in security hotspots that include the entire southern region of Mindanao, and help prevent cheating amid intense local political rivalries.

Two suspected grenade blasts were reported in the southern Maguindanao province, including one shortly before voting started at 6am. There were no reported injuries from the explosions.

Commission on Elections spokesman James Jimenez said the elections season has been peaceful so far compared to past years. Police say 20 people have died in poll-related violence, out of 76 reported casualties.

In Manila's financial district of Makati, former vice president Jejomar Binay protested after his ballot was rejected by an automated counting machine, one of several similar complaints. At least two mayoral candidates have been put in police custody elsewhere, including one for alleged elections fraud.

Opposition aspirants consider the Senate the last bastion of checks and balances given the solid dominance of Duterte’s loyalists in the lower House of Representatives. Last year, opposition senators moved to block proposed bills they feared would undermine civil liberties.

Duterte's politics and key programs, including his drive against illegal drugs that has left more than 5,200 mostly urban poor suspects dead, have been scrutinized on the campaign trail and defended by close allies running for the Senate, led by his former national police chief Ronald dela Rosa, who first enforced the crackdown when the president took office in mid-2016.

A May 3-6 survey by independent pollster Pulse Asia showed 11 of Duterte-backed senatorial candidates and four other aspirants in the winning circle, including only one from the opposition. The survey also showed some favourites had narrow leads and a considerable number of voters were undecided, indicating a probability the results could change. The survey of 1,800 respondents had a margin of error of plus or minus 2.3 percentage points.

Duterte himself remains hugely popular based on independent ratings surveys.