The party posed the most serious threat to Mr. Hun Sen’s Cambodian People’s Party, known as the C.P.P., and the C.N.R.P.’s dissolution cleared the way for Mr. Hun Sen’s party to sweep all 125 seats in the National Assembly in a 2018 election.
Mr. Kem Sokha’s arrest and the termination of the C.N.R.P. were part of a wide-ranging crackdown on opposition politicians, activists and members of the press that has seen hundreds of people jailed or sentenced in absentia after fleeing abroad. In June, a court in Phnom Penh convicted at least 51 opposition figures of “incitement” and “conspiracy” as well as other charges.
Among those convicted was Theary Seng, a lawyer and civil rights activist with dual American and Cambodian citizenship, who is now serving a six-year sentence in a remote prison in Preah Vihear Province.
Human Rights Watch, which has strongly condemned each step of the crackdown in Cambodia, called on foreign governments Friday to reassess their approach to Mr. Hun Sen’s government.
“It was obvious from the start that the charges against Kem Sokha were nothing but a political ploy by Prime Minister Hun Sen to sideline Cambodia’s major opposition leader and eliminate the country’s democratic system,” said Phil Robertson, deputy Asia director of Human Rights Watch.
He said the sentence “isn’t just about destroying his political party but about quashing any hope that there can be a genuine election in July.” Ming Yu Hah, Amnesty International’s deputy regional director over Southeast Asia, emphasized the same point, saying, “This verdict is an unmistakable warning to opposition groups months before national elections.”
Mr. Hun Sen put the point in graphic terms in a speech in January, in which he warned his political opponents to prepare for assault. He said he could “gather people belonging to the C.P.P. to protest and beat you,” and added, “Be careful. If I can’t control my temper, you will be destroyed.”
Sun Narin contributed reporting from Phnom Penh.
