The party of Cambodian Prime Minister Hun Sen declared a landslide victory in a general election on Sunday, a vote that critics widely dismissed as a sham aimed at cementing the party's rule before an expected transfer of power to his eldest son.

The contest was effectively a one-horse race, with Hun Sen's Cambodian People's Party (CPP), a political behemoth with a vast war chest, facing no viable opponent after a ruthless, years-long crackdown on its rivals.

Polls closed with a turnout of 84% according to the election committee, with 8.1 million people voting in a much-criticized contest between CPP and 17 mostly obscure parties, none of which won seats in the last election in 2018.

The only opponent with any real clout was disqualified from running.

"We've won in a landslide ... but we can't calculate the number of seats yet," said CPP spokesperson Sok Eysan.

Self-styled strongman Hun Sen, who has ruled Cambodia for 38 years, had brushed off all Western concern about the election's credibility, determined to prevent any obstacle in his carefully calibrated transition to his anointed successor and eldest son, Hun Manet.

No timeframe had been given for the handover until Thursday, when Hun Sen signalled his son "could be" prime minister next month, depending on "whether Hun Manet will be able to do it or not". He needed to win a National Assembly seat to become prime minister, which was likely.

Hun Sen said the turnout - the second highest in three decades - proved calls by his mostly overseas-based rivals to undermine the election with protest ballots had failed.