Cambodia's parliament on Tuesday approved a new five-year term for Prime Minister Hun Sen following weeks of political turmoil over his disputed election win.

The controversial move came despite recent mass public protests and a boycott of the National Assembly by the opposition, which has alleged widespread fraud in the July polls.

The opposition Cambodia National Rescue Party (CNRP) refused to take its seats for the opening session of the lower house on Monday, saying the kingdom was sliding towards "dictatorship".

Despite its absence, King Norodom Sihamoni asked Hun Sen's Cambodian People's Party (CPP) to form a new cabinet.

All 68 ruling party lawmakers on Tuesday approved a list of government members with Hun Sen reappointed premier through a show of hands, according to an AFP reporter in parliament.

The CNRP, which is demanding an independent investigation into the July elections, has described the legislature as a "one-party parliament".

It has rejected the results of the polls, alleging widespread vote irregularities.

"We are honest to the people. We are honest to the nation. We will never betray the will of the people," opposition leader Sam Rainsy said in a video clip released to coincide with the start of parliament on Monday.

According to official results, the CPP won 68 seats against 55 for the CNRP.

The number is enough for the ruling party to rubber-stamp the appointment of Hun Sen and his ministers and to pass legislation in the lower house.

But experts said the government would be seen as lacking political legitimacy if it introduces laws without an opposition in parliament.

"Hun Sen will have to be wary of potential unrest as many Cambodian people are viewing the CPP -- and Hun Sen himself -- as ruling without legitimacy," said Ou Virak, president of the Cambodian Center for Human Rights.

"Throughout these elections, we have seen the Cambodian people making their voices heard and demanding real change."

Tens of thousands of opposition supporters joined three days of demonstrations in the capital earlier this month. One protester was shot dead and several wounded as security forces clashed with a stone-throwing crowd.

Activists also accused dozens of police and thugs in civilian clothing of launching a violent crackdown on a peaceful vigil at a pagoda in the capital late on Sunday, using electric batons and slingshots.

Nine demonstrators as well as several foreign and local journalists were injured, witnesses said.

Hun Sen, who suffered his worst poll result in 15 years in July, last week agreed to find a peaceful solution to the dispute in talks with his main rival Rainsy. But he has ruled out an independent probe.

The 61-year-old former Khmer Rouge cadre -- who defected and oversaw Cambodia's rise from the ashes of war -- has vowed to rule until he is 74.

Garment exports and tourism have brought buoyant economic growth but Cambodia remains one of the world's poorer countries and the government is regularly accused of ignoring human rights and suppressing political dissent.

Younger Cambodians are also increasingly intolerant of endemic corruption and perceived social injustices including land grabs.