One of the world's longest-serving leaders, Cambodian Prime Minister Hun Sen, on Wednesday, said he would resign and hand power to his eldest son after almost four decades of ruling.

The former Khmer Rouge cadre has run the kingdom since 1985, eliminating all opposition to his power, with opposition parties banned, and challengers forced to flee and freedom of expression restricted.

His Cambodian People's Party (CPP) won a landslide victory in an election on Sunday with no meaningful opposition, taking 82 percent of the vote, paving the way for a dynastic succession to his eldest son that some critics have compared to North Korea.

"I would like to ask for understanding from the people as I announce that I will not continue as prime minister," the 70-year-old said in a special broadcast on state television.

Election authorities disqualified the only serious challenger, the Candlelight Party, on a technicality in advance of the election, as the party was expected to win all but five lower house seats.

The government hailed the 84.6 percent voter turnout as evidence of the country's "democratic maturity" but Western powers including the United States and European Union condemned the poll as neither free nor fair.

Hun Sen said Hun Manet, a 45-year-old four-star general, would take over as prime minister at the head of a new government on the evening of August 22.

"I ask people to support Hun Manet who will be the new prime minister," he said.