A Cambodian worker lies wounded during clash with military police during a garment workers' protest to demand higher wages in Phnom Penh on Jan. 3, 2014. Tang Shhin Sothy/AFP/Getty Images

Cambodian military police opened fire with assault rifles on Friday to quell a protest by stone-throwing garment factory workers demanding higher pay, killing at least four people, witnesses said.

Chaos during nationwide strikes erupted for a second day as security forces were deployed to break up the demonstration by thousands of workers, who refused to move and threw bottles, stones and petrol bombs outside a factory in Phnom Penh.

The clash represents an escalation of a political crisis in Cambodia, where striking workers and anti-government protesters have come together in a loose movement led by the opposition Cambodia National Rescue Party (CNRP).

Unions representing disgruntled garment workers have joined opposition supporters protesting against the government of Prime Minister Hun Sen to demand a re-run of an election in July that the opposition says was rigged.

Military police confronting the protesters fired live ammunition, Reuters journalists said, and bullet casings were later seen scattered across the ground at the scene.

Spokesmen for the national police and military police said they could not verify the number of casualties.

The incident followed a similar crackdown on Thursday in another part of the city, when armed troops struck demonstrators with batons. Witnesses said at least 20 people were wounded.

"If violence continues to happen and there are no talks, more violence will break out," said Chheng Sophors of the human rights group Licadho. "Protesters have become angry."

The CNRP, led by former finance minister Sam Rainsy, has courted some 350,000 garment workers from nearly 500 factories across the country by promising to nearly double the monthly minimum wage to $160 if it wins a re-run of the July election, which Hun Sen is refusing to hold.