Thousands of Cambodian opposition supporters on Sunday staged a fresh demonstration against Prime Minister Hun Sen's hotly disputed July election win, calling for a new poll to settle allegations of vote rigging.

An estimated 10,000 protesters, many waving Cambodian flags and banners, massed in a park in the capital Phnom Penh by Sunday afternoon demanding the strongman premier call a new election -- the first time they have made the demand.

The opposition Cambodia National Rescue Party (CNRP) party had previously asked for an independent probe into alleged vote rigging in July polls that extended Hun Sen's nearly three-decade rule.

Hun Sen's government has rejected a probe, prompting the opposition to call for elections.

There was a light security presence at the lively but peaceful rally -- the largest since October, according to an AFP reporter at the scene.

Protesters listened to an address by the opposition leader Sam Rainsy before beginning a march through the capital.

"We are demanding a re-vote because the previous election is not acceptable," Rainsy said to cheers.

"The (July) election is not fair... we cannot accept it," he added, before leading a chorus of "Hun Sen step down" with the throngs of flag-waving demonstrators, many wearing headbands reading "Long Live Democracy!" and "My Vote, My Life."

Rainsy's party has boycotted parliament in protest at alleged cheating in the July polls.

Sunday's rally was due to finish by nightfall but Rainsy said he would organise a new round of daily protests starting on Monday.

But some observers say the opposition protests are now largely symbolic, with months passing since the elections while Hun Sen remains firmly in charge.

"I think it would be hard to see what kind of concessions the protesters will get, given that the ruling party and Hun Sen are hardening their positions as the whole thing drags on," said Ou Virak, president of the Cambodian Center for Human Rights.

According to official results of the July polls, the ruling Cambodian People's Party (CPP) won 68 seats against 55 for the opposition.

In the aftermath of the disputed poll, opposition supporters took to the streets in demonstrations that left one protester dead and several injured after security forces clashed with a stone-throwing crowd.

Parliament in late September approved a new five-year term for Hun Sen, despite the absence of opposition MPs, in a move decried by the CNRP as a "constitutional coup".

A government spokesman dismissed the opposition's call for a new poll, saying it had failed to prove any voting irregularities.

"Thus CNRP is in no position to call for a new election. It just shows that CNRP is just a sore loser," Khieu Kanharith, who also represents the ruling CPP, told AFP.

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 has vowed to continue until he is 74.