The first of a promised three days of opposition protests in Cambodia's capital ended in violence Sunday, with several people injured and one person confirmed dead.

Tensions are high following July's disputed election, which both the ruling Cambodian People's Party and the opposition Cambodia National Rescue Party (CNRP) claim they won.

The first clash between protesters and hundreds of riot police took place shortly before 5 pm local time, along the riverside in Phnom Penh.

Police fired tear gas and water cannon, with protesters throwing bricks and metal bars.

Reporters saw one injured policeman and at least one injured protester taken away from the scene.

Cambodia's opposition leader Sam Rainsy addresses thousands of his supporters in the Cambodian capital, urging the crowd not to fight police. ( Robert Carmichael (@Carmichael_Rob) via Twitter )

The tense atmosphere calmed after opposition leader Sam Rainsy arrived on the scene, telling the crowd to stop fighting police and head back to Freedom Park - an open space in central Phnom Penh where tens of thousands of his supporters have rallied since early Sunday.

"Brothers, this is an important mission to rescue the nation," he said.

Many demonstrators held banners, with messages such as 'my vote, my nation' and 'where is my vote?'.

Mr Rainsy called for a recount or a new vote, but said there would be no talks on power sharing without the alleged election irregularities being cleared up.

One dead in clash

Late Sunday evening a number of witnesses confirmed that one opposition supporter had been shot during a clash on a major bridge in the capital.

President of the prominent Cambodian Center for Human Rights, Ou Virak, says he saw the man's body.

"I saw with my own eyes he is dead," he said.

"He was shot in the head... live ammunition must have been used."

It was not clear who had killed the man.

Mr Virak says at least ten other protesters have been injured.

Military police spokesman, Kheng Tito, says the military is unable to confirm the death and denies security forces have used live ammunition.

Journalists at the scene reported that police fired smoke grenades, rubber bullets and teargas at protesters, and also led several baton charges.

Much of central Phnom Penh has been sealed off with police roadblocks and razor wire since early Sunday causing traffic chaos.

Cambodian police reportedly refused to let an ambulance proceed past a barricade to a children's hospital.

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

On Monday, he will meet Mr Rainsy to discuss the growing political crisis.

Parliament is due to convene in a week, but the opposition has pledged to boycott the opening unless the ruling Cambodian People's Party agrees to an independent investigation into alleged widespread electoral fraud in the July 28 ballot.

The official results - which the opposition CNRP has rejected - show the ruling CPP having won 68 of the 123 seats in parliament and the CNRP with 55 seats.

ABC/AFP