PHNOM PENH (AFP) - Cambodian Prime Minister Hun Sen said on Thursday (Aug 2) opposition calls to boycott elections failed as he celebrated his "crystal clear" victory in the uncontested vote in a speech to thousands of garment workers.

Hun Sen's Cambodian People's Party (CPP) is poised to win all 125 parliamentary seats after the poll on Sunday, which critics have said was neither free nor fair. The result would see him extend his 33-year rule, cementing the country's status as a one-party state.

Opposition figures had called for a boycott of the vote with a so-called "sleep at home" or "clean finger" campaign after the most viable election contender, the Cambodia National Rescue Party (CNRP), was dissolved last year and barred from running.

"The sleep-at-home campaign has already failed. More than 82 per cent of Cambodians nationwide went to vote, it shows our Cambodians support the democratic process," he told a crowd of 14,000 workers in his first speech since the ballot.

"It is crystal clear that the future Prime Minister of the new government... is Hun Sen again," he said to applause.

He told the crowd that Cambodia's new Parliament would convene on Sept 19 and the government would be formed a day later.

Some 6.9 million votes were cast in Sunday's election - but nearly 600,000 ballots were spoiled, in a sign of unhappy voters.

'TERRIBLE SETBACK'

The United States and the European Union declined to send election observers to the poll because they said it lacked credibility with no viable opposition on the ticket.

Instead it was monitored by members of far-right and populist parties from Britain, Italy, Belarus and India.

The vote immediately drew condemnation from the EU and the US, while the self-exiled co-founder of the now disbanded CNRP Sam Rainsy told AFP this week it was a "terrible setback".

Rainsy, who lives in France to avoid a litany of charges he faces in Cambodia, also urged the international community to reject the results.

The CNRP, whose leader Kem Sokha is in jail on charges of treason, took 44 per cent of the vote in the last elections in 2013, the most credible challenge to Hun Sen's grip on on power in decades.

Hun Sen vowed on Thursday to double down on efforts to eliminate any remaining opposition in the country.

"(Opposition parties) deserve to die, if they're alive, war could erupt," he said before posing for selfies with smiling supporters.

The 65-year-old leader courted support from garment workers ahead of the election with his colourful speeches - and envelopes of cash.

He kept the tradition alive on Thursday handing each worker US$5 (S$6.83), and US$200 for pregnant employees.

The long serving leader is seen as a source of stability in a country with a tumultuous recent history of civil war and Vietnamese occupation.

He has also won support with ambitious infrastructure projects across the country, many funded with soft loans from China which provides support without asking questions about human rights.

Former allies Washington and the European Union both condemned the election this week, while China congratulated the people of Cambodia.