Cambodian opposition leader Kem Sokha was arrested in a pre-dawn raid at his home on Sunday after being accused by veteran prime minister Hun Sen of treason with the backing of the United States.
"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.
He said the foreign country involved was the United States.
Kem Sokha'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.
Hours after Kem Sokha's arrest on Sunday, the independent Cambodia Daily newspaper announced that it was closing from Monday after 24 years because of the government's demand for millions of dollars in back taxes.
Hun Sen, 65, has ruled Cambodia for more than three decades. A former Khmer Rouge cadre, he has become one of China's closest regional allies and has been making increasingly strident criticisms of the US.
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 him 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 legislators.
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.
Kem Sokha's daughter, Monovithya Kem, also a party official, said on Twitter that her father had been taken away handcuffed after a raid by between 100 and 200 police, who had arrived without an arrest warrant. She said his whereabouts were unknown.
Neither the US state department nor the White House responded immediately to a request for comment.
The Cambodian government has recently increased its rhetoric against the United States and last month ordered the expulsion of the US state department-funded National Democratic Institute pro-democracy group. Earlier in the year, it suspended joint military exercises with the United States, which has voiced fears over the human rights situation.
"Freedom of speech is rapidly becoming a highly endangered right in Prime Minister Hun Sen's march down the road to dictatorship in Cambodia," said Phil Robertson, deputy Asia director of the Human Rights Watch campaign group.
Last month, Hun Sen's government also stepped up attacks on the media, halting broadcasts by some radio stations and threatening to shut the Cambodia Daily over what it said were $6.3 million (Dh23.25m) in unpaid taxes.
The newspaper said Monday's edition would be its last and blamed "extra-legal threats by the government to close the Daily, freeze its accounts and prosecute the new owner" for the closure.
During Hun Sen's rule Cambodia emerged from the devastating Khmer Rouge genocide to enjoy record years of economic growth of above 7 per cent, but disaffection has been growing and he only just won the 2013 election against a unified opposition.
His Cambodian People's Party won local elections in June, but the opposition also did well, increasing expectations of a close contest in the general election due in 2018.
Kem Sokha took over the party leadership after his predecessor, Sam Rainsy, resigned in February.
Sam Rainsy lives in exile in France to avoid a defamation conviction he says was politically motivated.
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