At least four Cambodians were killed when police opened fire on striking garment workers, as opposition-backed protesters pressed their calls for wages to be doubled.

An Associated Press photographer and human rights workers said police fired assault rifles on Friday, after several hundred workers blocking a road south of the capital Phnom Penh began burning tires and throwing objects at them. Several wounded workers could be seen after the shots were fired.

Phnom Penh deputy police commissioner Chuon Narinhas said that four people have been killed.

The local human rights group LICADHO also said in a statement that at least four civilians were shot dead and 21 injured in what it described as "the worst state violence against civilians to hit Cambodia in 15 years."

The local human rights group LICADHO said in a statement that at least four civilians were shot dead and 21 injured in what it described as "the worst state violence against civilians to hit Cambodia in 15 years."

The statement said that security forces used live ammunition to shoot directly at civilians.

"The use of live ammunition was prolonged and no efforts appear to have been made to prevent death and serious injury," it said.

"Reports suggest that security forces were also injured after being hit with stones."

It was not clear whether those killed were workers or local residents who had joined in the protest.

"They are anarchists, they have destroyed private and state property,'' Chuon Narin, the deputy police chief, told the Associated Press news agency by phone. "That is why our forces need to chase them out."

About 500,000 Cambodians are employed in the garment industry, which is worth $5bn a year to the economy in exports. The government has offered $100 as a minimum monthly wage, short of a $160 wage pledged by the opposition Cambodia National Rescue Party.

The clash comes a day after Cambodian soldiers forcefully quelled a demonstration, a turning point after two weeks of relatively peaceful strikes, marches and demonstrations in Cambodia.

The opposition Cambodia National Rescue Party has called for Prime Minister Hun Sen to step down, and for new elections. Hun Sen has rejected the demands.

Hun Sen won elections last July that extended his 28-year rule, but protesters led by opposition head Sam Rainsy accuse him of rigging the vote.

Although the wage and election issues are not directly linked, Cambodia's opposition has had long and close ties with the country's labour movement.