Cambodian anti-riot police stopped hundreds of people from protesting at a court where a leading government critic went on trial Tuesday for an alleged secessionist plot, according to an AFP photographer.
Mam Sonando, owner of the independent Beehive radio station, was arrested in mid-July on charges including insurrection and inciting people to take up arms against the state.
He is accused of masterminding a plot to establish an autonomous region in eastern Kratie province -- the scene of violent land rights protests -- and faces a maximum of 30 years in prison if convicted.
A teenage girl was shot dead in Kratie in May when security forces clashed with demonstrators.
Sonando, the 71-year-old president of campaign group the Association of Democrats, denies the accusations.
Around 500 people attempted to rally outside the Phnom Penh court as the hearing began but authorities and anti-riot police sealed off the building.
Protesters, many carrying banners calling for Sonando's release, briefly clashed with police, according to an AFP photographer and witnesses.
Touch Naruth, Phnom Penh police chief, told AFP the protesters were not allowed to rally because the noise would "bother the hearing".
Journalists and civil society activists were also barred from attending the hearing, witnesses at the court said.
Sonando was arrested previously in 2003 and 2005 for his political activities and defaming Prime Minister Hun Sen's government.
The 60-year-old PM has ruled Cambodia since 1985 and has vowed to remain in power until he is 90 years old.
Sonando's detention in July came just two days after US Secretary of State Hillary Clinton concluded an official visit to Cambodia.
Activists say land conflicts are Cambodia's most pressing human rights issue. Protests have intensified this year and campaigners say authorities are increasingly cracking down on dissent.