Over 14 million voters in Malaysia are heading to the polls in an election that will see the incumbent leader facing off against his 92-year-old former mentor.
Opposition leader Mahathir Mohamad will be up against current PM Najib Razak’s ruling Barisan Nasional (BN) coalition.
An opposition victory would be unprecedented in Malaysia, where BN has ruled for 61 years.
But critics have raised concerns the election will not be free and fair.
Malaysian politics have been dominated since independence by the BN coalition and its major party, the United Malays National Organisation (UMNO).
But the coalition has been losing ground over recent general elections.
In the last election, in 2013, the opposition made unprecedented gains – winning the popular vote – although it failed to win enough seats to form a majority government.
Afterwards, then opposition leader Anwar Ibrahim was sentenced to five years jail on sodomy charges – charges he claimed were part of a political smear campaign.
Former prime minister Mr Mahathir, the country’s longest running prime minister who governed between 1981 and 2003, was previously an integral part of BN and a mentor to Mr Najib.
But he abandoned the coalition in 2016 saying he was “embarrassed” to be associated with a party “that is seen as supporting corruption”. And in January he announced he was running as leader of the opposition coalition Pakatan Harapan.
Mr Najib has been accused of pocketing some $700m from the 1Malaysian Development Berhad, a state investment fund.
He has vehemently denied all allegations and been cleared by Malaysian authorities.
The fund is still being investigated by several countries and Mr Najib has been accused of stifling Malaysian investigations by removing key officials.
The government recently passed a law redrawing election boundaries, leading to accusations that is had gerrymandered constituencies to ensure they were filled Malay Muslims, who are traditionally BN supporters.
In the days before the poll, election reform group Bersih 2.0 accused the Election Commission (EC) of multiple “electoral crimes” including irregularities in postal voting and failing to remove dead people from the electoral roll.
A controversial fake news law was also recently introduced, which critics say could be used by the authorities to muffle dissent.
Dr Mahathir is himself being investigated under the law after alleging that his plane had been sabotaged.
The government has insisted the election will be free and fair, with Mr Najib saying that the EC acted “for the good of all”.
Voters will elect 222 members of parliament as well as state assembly members in 12 of the 13 states.
Malaysia uses a first-past-the-post electoral system, whereby the party that gets the most seats in parliament wins even if it does not get the popular vote.