Violence erupted after around 300 opposition activists protested outside Phnom Penh's "Freedom Park" which has been closed since a bloody crackdown on striking garment workers in January.
Scores of security guards and protesters fought with sticks and stones injuring atleast 30 people, mostly guards, said Chan Soveth of the Cambodian Human Rights and Development Association (ADHOC).
An AFP photographer at the scene saw at least two guards lying on the ground bleeding with head wounds.
It was the latest opposition rally in Cambodia where authorities have responded violently to any form of public dissent in recent months.
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In January police shot dead four striking garment factory employees for demanding higher wages in what activists described as the country's worst state violence against its citizens in 15 years.
Military police spokesman Kheng Tito told AFP that 17 guards were injured in today's clashes and three opposition politicians, including prominent MP-elect Mu Sochua, were detained for leading the protest.
Yim Sovann, spokesman for the opposition Cambodia National Rescue Party, demanded authorities release his colleagues, saying their actions were in line with the constitution.
"Authorities should not have closed Freedom Park because it is the only place for people in Phnom Penh and other provinces to protest," he said.
The park was launched in 2010 as a designated area for people to air their grievances, similar to Speakers' Corner in London's Hyde Park. But it has been sealed off with razor wire barricades since May.