PHNOM PENH: Violent clashes erupted in the Cambodian capital on Sunday, leaving one protester dead and several wounded as thousands gathered to challenge premier Hun Sen's disputed election win, witnesses said.

Security forces fired smoke grenades, tear gas and water cannon at rock-throwing opposition supporters in Phnom Penh in an escalation of the political crisis that has gripped the kingdom since controversial July polls.

In one of a series of clashes around the city, anti-riot police baton charged a group of protesters who were throwing rocks near an overpass some five kilometres (three miles) from the main protest site, according to witnesses.

One demonstrator was killed, according to prominent activist Ou Virak, president of the Cambodian Center for Human Rights.

"I saw with my own eyes he is dead. He was shot in the head," he told AFP, adding that it was unclear who fired the shot.

At least 10 other protesters were injured, he added.

Military police spokesman Kheng Tito said he was unable to confirm the death. He denied the security forces had fired live ammunition.

"The military police used only batons and shields and police used tear gas. We did not use live rounds," he said.

The violence came after an estimated 20,000 supporters of the opposition Cambodian National Rescue Party (CNRP) gathered in a park in the capital demanding an independent probe into the election.

"Our votes were robbed," said 56-year-old Srin Chea, who travelled from southern Kandal province.

"I am angry. I want justice. I am not afraid of death."

Hun Sen, 61, has been in power for 28 years and has vowed to rule until he is 74.

A former Khmer Rouge cadre who defected and oversaw Cambodia's rise from the ashes of war, his government is regularly accused of ignoring human rights and suppressing political dissent.

Opposition chief Sam Rainsy addressed thousands of cheering supporters at the main protest site, calling for a recount or new vote.

He said opposition lawmakers would boycott the opening of the parliament on September 23.

"There will be no talks on power sharing" without the alleged election irregularities being cleared up, he warned.

"Brothers, this is an important mission to rescue the nation," Rainsy told demonstrators, many of whom held banners reading 'my vote, my nation' and 'where is my vote?'.

The violence came despite a rare meeting on Saturday between Rainsy and Hun Sen, hosted by King Norodom Sihamoni, that was later described by the opposition leader as a "first step" towards ending the crisis.

Further talks between the two main political parties were due to take place on Monday.

Ahead of Sunday's rally the government set a limit on the number of protesters at 10,000 and said the gathering must finish by around 6pm local time (1100 GMT).

But thousands of demonstrators remained into the night ahead of the second of three days of planned rallies, vowing to stay until their demands are met.

According to final election results, Hun Sen's Cambodian People's Party (CPP) won 68 seats in the July polls against 55 for the opposition.

The CNRP has rejected the tally, alleging widespread vote rigging, but so far its efforts to challenge the results have failed and it has few formal options left in its bid to overturn Hun Sen's victory.