PHNOM PENH – Thousands of Cambodia opposition supporters staged a demonstration Wednesday over disputed elections that extended strongman Prime Minister Hun Sen's near three-decade rule, under a heavy security presence after bloody clashes last month.

Crowds filled a downtown park to hear Sam Rainsy, whose opposition Cambodia National Rescue Party (CNRP) has boycotted parliament over the controversial July polls.

"We ask you to participate in the demonstration to protect our nation and seek justice for voters," he told supporters, reiterating a claim that his party had won the election.

An AFP reporter at the scene said around 10,000 people had packed into the park for the rally, which appeared to be passing off peacefully.

The party has vowed to work with authorities to stop any clashes.

"It will be completely peaceful... If there is any violence, it would not come from us," Rainsy said at a press conference earlier Wednesday, adding that the protest would last for three days.

Thousands of riot police were deployed along the streets and at significant locations in Phnom Penh to meet the first major show of strength by the opposition since tens of thousands of its supporters joined three days of rallies in the capital in September.

Those demonstrations left one protester dead and several wounded after security forces clashed with a stone-throwing crowd.

The opposition has demanded an independent investigation into allegations of electoral fraud over the polls, which plunged the country into political turmoil.

Rainsy, who travelled to the United States this month as part of efforts to raise his complaints with the international community, said the opposition would not back down.

"We cannot bury the irregularities... we cannot move on properly unless they are resolved," he said, repeating allegations that Hun Sen's ruling Cambodian People's Party (CPP) had "robbed" votes.

Many of the protesters wore ribbons tied around their heads carrying slogans such as "Where is my vote?", "We need a truth committee", and "Long live democracy!".

"I am protesting to demand my vote back, they stole my vote. I want justice. If they do not return my vote, I will protest until the current government collapses," 72-year-old demonstrator Phay Math told AFP.

Riot police have held public training sessions in a central Phnom Penh park in recent days.

"We will intervene immediately when there is any problem," said Kheng Tito, spokesman for the military police, told AFP.

Cambodia's parliament in late September approved a new five-year term for Hun Sen, despite the absence of Rainsy's party, in a move decried by the opposition as a "constitutional coup".

The opposition has said they had collected around two million thumbprints to go with a petition calling on the UN and foreign powers to intervene after the disputed polls.

Protesters are due to march to the United Nations human rights office in the capital later Wednesday to deliver the petition.

They also plan to take the document to a number of foreign embassies, including those of France, the United States, Britain and China over coming days.

Hun Sen – a 61-year-old former Khmer Rouge cadre who defected and oversaw Cambodia's rise from the ashes of war – has ruled for 28 years and vows to continue until he is 74.