The arrest of Kem Sokha during a midnight raid on his Phnom Penh home appeared to be part of a broader push by the government of Prime Minister Hun Sen, one of the world's longest serving rulers, to crush opponents and silence critics ahead of the vote.
The government said it had evidence that Mr. Kem Sokha was involved in a secret plot and had conspired with foreigners to harm the country.
Pictures in Cambodian media showed Sokha being led away with his hands behind his back.
"Kem Sokha and all bodyguards are taken away by 100-200 police without warrant after they raided his home".
Khieu Sopheak said that Kem Sokha had admitted in the video that "he was trained and received funding from a powerful foreign country to topple the government".More news: Japan confirms North Korea has conducted sixth nuclear test
"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. Probably, that video is the only way to nail opposition leader down.
Charles Santiago, a member of the Malaysian Parliament and chairman of the organization, said the arrest violated the immunity protection that should be afforded Mr. Kem Sokha as a member of Cambodia's Parliament. The CNRP released a statement condemning the arrest of their leader and called for Sokha's unconditional release.
Sokha's daughter who is a party official wrote. She said later that his whereabouts were unknown.
FILE PHOTO - Cambodia's Prime Minister Hun Sen smiles as he arrive at the National Assembly of Cambodia during a plenary session, in central Phnom Penh, February 20, 2017.
Kem Sokha made no immediate comment and it was not clear if he had been charged or had legal representation at this stage.More news: Canadian GDP sees strongest half-year growth in 15 years: StatsCan
Neither the US State Department nor the White House responded immediately to a request for comment. Earlier in the year, it suspended joint military exercises with the United States, which has voiced fears over the human rights situation.
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.
Last month, the government stopped broadcasts by some radio stations and ordered an independent newspaper, The Cambodia Daily, to pay $6 million in tax, or face closure.
Mr Hun Sen added that the national elections would proceed as usual.More news: Trump says he will fix 'massive trade deficit' between U.S. and Ireland