Cambodian opposition leader Kem Sokha has been arrested in a police raid on his home and veteran Prime Minister Hun Sen is accusing him of treason with the backing of the United States.
Sunday's arrest marks an escalation in a campaign against critics, independent media and any potential threats to Hun Sen's hold on power ahead of an election next year at which Kem Sokha has been expected to be his main challenger.
"It's an act of treason with conspiracy with a foreign country, betraying his own nation. This requires arrest," Hun Sen told a group of garment workers according to the pro-government Fresh News website.
Hun Sen said of the alleged foreign third party: "It's the United States."
Hun Sen, 65, has ruled the Southeast Asian country for more than three decades. The former Khmer Rouge cadre has become one of China's closest regional allies and has been making increasingly strident verbal attacks on the United States.
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Kem Sokha, 64, has led the main opposition Cambodia National Rescue Party (CNRP) since his predecessor resigned in February, saying he feared a government plan to shut it down.
Pictures in Cambodian media showed Sokha being led away with his hands behind his back.
The government released a video on its Facebook page in which Kem Sokha appeared to tell a group of supporters about a strategy to win power which he said had the support of "the Americans" rather than an immediate plot to topple Hun Sen.
In the video, which the government said was shot in Australia, Kem Sokha said the Americans had hired academics to advise on strategy to change Cambodia's leaders.
"And if I follow such a tactic and strategy and if I could not win, I do not know what else to do," he said.
The opposition party made no immediate comment on the veracity or content of the video. Earlier, it said Kem Sokha's arrest was politically motivated and violated the law because of the immunity granted to elected mps.
The party called for his release and urged the international community to intervene. It wanted a non-violent approach, it said.
If Kem Sokha is found guilty of any offence, it could allow the government to shut the party under a new law that forbids parties from having a leader who has been convicted.
