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.
"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, with the opposition Cambodia National Rescue Party holding daily protests calling 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 Asian nation, but opposition protesters accuse him of rigging the vote. Hun Sen has rejected their demand.
Although the wage and election issues are not directly linked, the opposition has close ties with the country's labor movement. Last 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 2012, Cambodia shipped more than $4 billion worth of products to the United States and Europe.
The site of Friday's clash -- a street with stores and dwellings -- was strewn with debris. Walls bore holes apparently caused by bullets.
Mak Vin, a 25-year-old worker, said he was among those protesting for more than a week over the wage issue. He said that on Friday morning, as the workers burned car tires and shouted slogans, "hundreds" of armed police arrived and opened fire.
"They fired live bullets directly at us. I am very scared," Mak Vin said.
There had been an earlier clash overnight, with no known fatalities.
Mak Vin said the workers were protesting only for higher wages, and would return to work once that demand was met. He said most workers were not cowed by the shooting, and would continue their strike.
One measure of the seriousness of the situation was an unusual statement issued by the Defense Ministry affirming the military's loyalty to the government. The statement said the army would take whatever action was necessary to defend the legal government, the king and the constitution.
Friday's confrontation followed a similar 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.
Violent suppression of social and political protests has not been unusual under Hun Sen's authoritarian government, but there have been few incidents in recent years where more than one person has been killed.
The authorities also usually shy away from using live ammunition in Phnom Penh, where the population is largely hostile to the government.
But the U.N. special rapporteur on human rights in Cambodia, Surya Subedi, said it was the third time since the disputed elections that authorities have shot into a crowd and caused fatalities. He called for an independent investigation into whether excessive force was used. He also expressed concerned about increasing violence by some demonstrators.
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 exports fuel the economy and who are generally seen as financial supporters of Hun Sen's ruling Cambodian People's Party.
Last week, the Garment Manufacturers Association in Cambodia called for factory owners to close their plants, ostensibly for fear of damage by protesters. The situation puts pressure both on the striking workers, who are not being paid, and the government, which relies on garment exports to power the economy.
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