Hun Sen was quoted as telling a group of garment factory workers that the "third party" behind the plot was the same as that behind a 1973 coup that is widely believed to have had US support.
Sam Rainsy has been living in self-exile in France since 2015 after facing a lawsuit filed against him by Prime Minister Hun Sen.
The government said it had obtained a video clip and other evidence indicating "secret plans of a conspiracy between Kem Sokha. and foreigners to harm the Kingdom of Cambodia".
CNRP President Mr Sokha was arrested in the early hours of Sunday on allegations of treason and was sent to be detained in a prison in Kbong Khmum province, which is about 80 kilometres from Phnom Penh.
Sokha reportedly says in the purported video that "he was trained and received funding from a powerful foreign country to topple the government". Kem Sokha has a long, distinguished, and internationally recognized commitment to human rights and peaceful democracy.
The opposition party made no immediate comment on the veracity or content of the video.
Sokha's daughter who is a party official wrote.
Kem Sokha made no immediate comment and it was not clear if he had been charged or had legal representation at this stage.
His Cambodian People's Party won local elections in June, but the opposition Cambodia National Rescue Party (CNRP) also did well, increasing expectations of a close contest in the coming general election.
The government has stepped up attacks on the United States and last month ordered the expulsion of the U.S. State Department-funded National Democratic Institute pro-democracy group. Her statement did not directly address Hun Sen's accusations against the United States.
This government move follows a number of troubling recent steps, including the imposition of unprecedented restrictions on independent media and civil society.
An independent newspaper, The Cambodia Daily, has been ordered to halt publishing by Monday because of a tax dispute, and at least 15 radio stations have been ordered to stop broadcasting programming from the Voice of America and Radio Free Asia.
The English-language paper, founded by an American journalist, was known for critical coverage of issues such as corruption, human rights and the environment.
Mr Hun Sen added that the national elections would proceed as usual.