Cambodian opposition leader Kem Sokha was arrested in Phnom Penh early on Sunday and the government of veteran Prime Minister Hun Sen said he was accused of treason.
The government said in a statement it had a video clip and other evidence that pointed 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.
Tension has been growing in the Southeast Asian country in the run-up to a general election next year in which the opposition party has been expected to pose a strong challenge to Hun Sen's rule of more than three decades.
Kem Sokha's daughter Monovithya Kem, who is also an official in his Cambodia National Rescue Party (CNRP), said on Twitter that Kem Sokha and his bodyguards had been taken away.
"Kem Sokha is in police custody at city hall. Handcuffed. No arrest warrant. After 100-200 police raided and vandalised his home," she said.
Fresh News, a pro-government website, earlier said it had video of Kem Sokha discussing overthrowing Hun Sen with support from the United States.
It showed a picture of him being led away in handcuffs.
Kem Sokha made no immediate comment and it was not clear if he had legal representation at this stage.
Last month, Hun Sen's government stepped up attacks on the media, halting broadcasts by some radio stations and ordering an independent newspaper, The Cambodia Daily, to close if it did not pay a $6m tax bill within days.
The government also increased its rhetoric against the United States and ordered the expulsion of the US State Department-funded National Democratic Institute pro-democracy group.
Hun Sen's Cambodian People's Party won local elections in June, although the opposition Cambodia National Rescue Party fared reasonably well, increasing expectations of a close contest in the general election.
Kem Sokha took over the party leadership after his predecessor, Sam Rainsy, resigned in February. Rainsy said he 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.
Rainsy lives in exile in France to avoid a defamation conviction he says was politically motivated.
Source: Reuters news agency