Opponents accuse Prime Minister Hun Sen of using unfair measures to undermine rivals in the run-up to local elections next month and a general election next year.
Cambodia's Prime Minister Hun Sen warned on Wednesday that protests against him would not be tolerated and quashed with military force on the eve of local elections slated for next month.
His warning referred to opposition protests that broke out during the last general election in 2013, when a crowd destroyed at least two police cars over allegations of voter fraud.
Hun Sen is one of the world's longest serving rulers after more than three decades in power. He is seeking a new term in office in the 2018 national election. His opponents have accused him of using unfair measures to undermine rivals in the run-up to the elections.
Human rights groups say more than a dozen activists and opposition politicians have been thrown in jail since the 2013 election that challenged the ruling party's majority.
Speaking at the inauguration of a government office in Tbong Khmum, Hun Sen, dressed in a military uniform, said he would not hesitate to lead a civil war if the protests turned violent.
"Remember, in the 2017 and 2018 [elections], if your group does such activities again, armed forces will crack down on them immediately. If war happens, let it be," the long-serving leader said.
"I still insist that war will happen if the [ruling] Cambodian People's Party isn't in power," he added.
The former soldier first took office in 1985, when Cambodia was emerging from the shadows of the Khmer Rouge regime.
Hun Sen has dominated the country ever since, amassing extensive control over its judicial system, security forces and economy.
However, there is growing disillusionment at the corruption and political repression associated with Hun Sen's rule, as well as his family's wealth.