The party of long-serving Cambodian Prime Minister Hun Sen declared victory in a general election on Sunday, a contest that had been widely dismissed as a one-sided sham aimed at cementing the party's decades-long rule.

"We've won in a landslide ... but we can't calculate the number of seats yet," said Sok Eysan, spokesperson for the Cambodian People's Party (CPP), which ran virtually unopposed.

The 70-year-old Hun Sen, a former Khmer Rouge cadre, has ruled since 1985 and faced no real contest in the vote, with opposition parties banned, challengers forced to flee and freedom of expression stifled.

The CPP is expected to retain all 125 seats in the Lower House, prolonging his grip on power and paving the way for him to hand the reigns to his son in a dynastic succession some critics have compared to North Korean politics.

The only serious opposition party was disqualified on a technicality in the run-up to the polls and it will be a surprise if any of the 17 other small, poorly funded parties win seats.

More than 9.7 million people were registered to vote in Cambodia's seventh election since the United Nations first sponsored polls in 1993, after years of conflict — including the era of the genocidal Khmer Rouge — left the country devastated.

Over the last 30 years, whatever hopes the international community might have had for a vibrant multiparty democracy in Cambodia have been flattened by the juggernaut of Hun Sen's rule.