At least four people were killed Friday when police outside Cambodia's capital opened fire to break up a protest by striking garment workers demanding a doubling of the minimum wage, police and human rights workers said.

Chuon Narin, deputy chief of the Phnom Penh Municipal Police, said the four were killed and about 20 others wounded in a southern suburb of the capital after several hundred workers blocking a road began burning tires and throwing objects at police officers. Witnesses said some officers fired AK-47 rifles into the air and that others shot at ground level.

Workers at most of Cambodia's more than 500 garment factories are on strike, demanding an increase in the minimum wage to $160 a month, double the current rate. The government has offered $100 a month.

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.

Cambodia Labor Unrest

A villager checks a body after the man was shot dead, as riot police opened fire on garment workers near a factory on the Stung Meanchey complex on the outskirts of Phnom Penh on Jan. 3, 2014. (Heng Smith/Associated Press)

"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, said by phone. "That is why our forces need to chase them out."

The protesters were cleared from the street, at least temporarily, by early afternoon.

The violence comes at a time of political stress in the country, as the opposition Cambodia National Rescue Party has protested daily for Prime Minister Hun Sen to step down and call elections. Hun Sen won elections last July that extended his 28-year rule in the poor Southeast Asia nation, but opposition protesters accuse him of rigging the vote. Hun Sen has rejected their demand.

Most garment workers on strike

Workers at most of the country's more than 500 garment factories are on strike, demanding an increase in the minimum wage to $160 a month, double the current rate. The government has offered $100 a month.

Although the wage and election issues are not directly linked, the opposition has close ties with the country's labour movement. On Sunday, many workers joined a massive political rally organized by the opposition.

The workers represent a potent political force, because the garment industry is Cambodia's biggest export earner, employing about 500,000 people in garment and shoe factories. In 2012, the Southeast Asian country shipped more $4 billion worth of products to the United States and Europe.

Friday's confrontation followed a similar violent one a day earlier at a different location, in which elite troops broke up a demonstration outside a factory, beating demonstrators and arresting 10 people, including Buddhist monks, according to witnesses from human rights groups.

In that case, according to the local human rights group LICADHO, "The soldiers were seen brandishing metal pipes, knives, AK-47 rifles, slingshots and batons."

The standoff over wages presents Hun Sen with a dilemma, as increasing violence could drive the workers into a tighter alliance with the opposition, providing a vast pool of people for their increasingly confident street demonstrations. But the government is also close to the factory owners, whose export products are the locomotive for the economy.

The Garment Manufacturers Association in Cambodia last week called for factory owners to close their plants, ostensibly for fear of damage by protesters. The situation puts pressure both on the workers, who go without pay, and the government, which relies on garment exports to power the economy.

In an evident effort to increase the pressure on Hun Sen, the association on Thursday sent a letter to the government asking that their members be allowed to export capital equipment to other countries because they were unable to operate in Cambodia. There was no immediate response from the government.