Cambodian human rights groups said at least 40 people were injured Tuesday during clashes between security guards and opposition protesters in Phnom Penh.
The protesters were demanding the government re-open Freedom Park, the capital's main protest site, which has been closed since political violence in January.
Rights groups said when the protesters tried to attach a banner to a barricade surrounding the park, civilian security officers moved in and began beating several demonstrators.
The demonstrators then turned on the security guards, in some cases beating them with their own batons, leaving several bloodied.
City officials said at least 37 guards were injured.
Police used tear gas to disperse the protest. They arrested at least three politicians accused of leading the group, which was estimated at between 200 and 500 people.
Freedom Park, which serves as a designated area for demonstrations, has been closed since January, when police shot and killed five striking garment factory employees.
Since then, activists said security guards, who are hired by local authorities, have repeatedly beaten protesters who attempted to gather outside the park.
Following the January unrest, Cambodia imposed a ban on public protests. The ban has since been lifted, but it has been difficult to get permission for demonstrations.
Tuesday's demonstration was organized by the opposition Cambodia National Rescue Party, or CNRP, whose members have refused to take their seats in parliament in protest at an election last year they say was rigged.
Cambodian Prime Minister Hun Sen, who has ruled the country since 1985, said the election was fair.