Thousands of Cambodian opposition supporters protested 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 central park to hear a speech by Sam Rainsy, whose opposition Cambodia National Rescue Party (CNRP) has boycotted parliament in protest at alleged cheating in the 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.
Thousands of demonstrators then walked through the streets of the capital for around an hour, with many waving flags and chanting "Change! Change!"
Protesters marched to the United Nations human rights office in the capital, where they deliver a petition calling on the UN and foreign powers to intervene.
The opposition has said it has collected around two million thumbprints on the document, and boxes of these were unloaded at the UN late Wednesday.
"I hope the UN will help find justice for all of us. Our votes were stolen," protester Sa Pha, 40, told AFP.
Police kept a low profile around the main protest areas.
But thousands of riot police were deployed at main 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 vowed to work with authorities to stop any clashes this time.
"It will be completely peaceful... If there is any violence, it would not come from us," Rainsy said earlier Wednesday, adding that the protest would last for three days.
Riot police have held public training sessions in a central Phnom Penh park in recent days.
The opposition has demanded an independent investigation into allegations of electoral fraud over the polls, which plunged the country into political turmoil.
The party plans to take its petition to a number of foreign embassies, including those of France, the United States, Britain and China over coming days.
Rainsy, who travelled to the US 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 headbands bearing slogans such as "Where is my vote?", "We need a truth committee" and "Long live democracy!".
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".
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.