The UN rights chief voiced alarm Monday at the arrest of Cambodian opposition leader Kem Sokha for alleged treason, warning of the risk he may not get a fair trial.
Sokha, the leader of the main opposition Cambodia National Rescue Party (CNRP), was arrested early Sunday, marking the latest in a flurry of legal cases lodged against critics and rivals of strongman premier Hun Sen.
The arrest "appears to have been carried out with no respect for due process guarantees, including respect for his parliamentary immunity," Zeid Ra'ad Al Hussein said in a statement.
Sokha's daughter Kem Monovithya described on Twitter how her father was handcuffed and "taken away by 100-200 police without (a) warrant after they raided his home."
On Saturday night, a pro-government website -- Fresh News -- alleged that Kem Sokha had discussed overthrowing Hun Sen with support from the United States.
It did not provide any evidence for the claim.
Zeid said the treason allegations against Sokha were based on a video of a speech he made in 2013, which had been publicly available ever since.
If found guilty, the opposition leader risks a prison sentence of between 15 and 30 years.
"I am also concerned that numerous public statements by the prime minister and high-ranking officials about Sokha's supposed guilt breach the presumption of innocence and the right to a fair trial to which he is entitled under Cambodian and international human rights law," Zeid said.
He said the arrest was "all the more worrying as it takes place amid other recent (government) measures," including the closure of one of Cambodia's few remaining independent newspapers.
The southeast Asian country has been run for more than three decades by 65-year-old Hun Sen, a wily political operator who has long used the courts and strongarm tactics to silence critics.
He faces a key test at national polls next year with the CNRP gaining in popularity amid mounting anger over corruption and inequality.