Cambodian opposition leader Kem Sokha has been arrested at his home in the capital Phnom Penh, with the Government of veteran Prime Minister Hun Sen saying he was accused of treason.
The arrest marks a new 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.
The Government said it had a video clip and other evidence pointing to "secret plans of conspiracy between Kem Sokha, others and foreigners to harm the Kingdom of Cambodia".
"The above act of this secret conspiracy is treason," the statement said.
The Australian Government called for Cambodia to handle the matter in a transparent manner.
"The Australian Government is concerned by the arrest of Cambodian opposition leader Kem Sokha overnight," a Department of Foreign Affairs and Trade spokesperson told the ABC.
"We urge Cambodian authorities to handle the matter in an open and transparent manner, and to take all necessary steps to maintain an open democratic space in which all voices can be heard"
Hun Sen, 65, has ruled the South-East 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.
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.
Kem Sokha's daughter, Monovithya Kem, who is also an official in the party, said on Twitter that her father had been taken away handcuffed after a raid by between 100 and 200 police, who arrived without an arrest warrant.
"Kem Sokha whereabouts is still unknown," she wrote, after earlier saying he had been taken to city hall.
Kem Sokha made no immediate comment and it was not clear if he had legal representation at this stage.
Government accuses US of supporting Kem Sokha
Fresh News, a pro-government website, had said before Kem Sokha's arrest that it had video of Kem Sokha discussing overthrowing Hun Sen with support from the United States.
Neither the US State Department nor the White House responded immediately to a request for comment.
The Government has recently increased its rhetoric against the US and last month ordered the expulsion of the US State Department-funded National Democratic Institute pro-democracy group.
"Freedom of speech is rapidly becoming a highly endangered right in Prime Minister Hun Sen's march down the road to dictatorship in Cambodia," deputy Asia director of the Human Rights Watch campaign group, Phil Robertson, said.
"Kem Sokha is now the latest victim," he added, calling on donors to condemn the arrest.
Charles Santiago, Malaysian MP and chairman of the ASEAN Parliamentarians for Human Rights, added: "The arrest of opposition leader Kem Sokha early this morning takes the ongoing crackdown by the ruling party in Cambodia to an alarming new level.
"With national elections on the horizon, it is clear that this is nothing more than a thinly veiled attempt to crush the opposition before the campaign even starts.
"For months, we have been witnessing the escalation of Government attempts to cripple the opposition, but it appears now that Prime Minister Hun Sen is so afraid of what might happen in a genuine vote, he won't allow for competition at all."
Hun Sen's Government stepped up attacks on the media last month, halting broadcasts by some radio stations and ordering an independent newspaper, The Cambodia Daily, to close if it did not pay a $6 million tax bill within days.
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 also won local elections in June, but the opposition also fared reasonably 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.
Mr Rainsy said he had quit to save the party in the face of a threatened ban on any party with a leader who has been convicted of a crime.
Mr Rainsy lives in exile in France to avoid a defamation conviction he says was politically motivated.