At least 30 opposition activists have been arrested this year and accused of plotting to overthrow the government before the planned return from self-exile of Sam Rainsy, founder of the dissolved Cambodia National Rescue Party (CNRP), on November 9.

In a ceremony on Monday in the capital, Phnom Penh, Prime Minister Hun Sen said Rainsy's return would be an incursion by forces seeking to overthrow his government.

Rainsy has called for an uprising against the longtime leader. Hun Sen said that if opposition leaders and supporters returned, such declarations would mean "the army must begin to deploy and use weapons of all kinds".

"Attack wherever they are seen, there is no need to wait for an arrest warrant or not," he said.

"Supporters are arrested whenever they are seen."

In 2018, Hun Sen extended his rule of more than three decades in an election in which his ruling party won all the seats in parliament.

The CNRP had been disbanded months before the election by the Supreme Court, following the 2017 arrest of party leader Kem Sokha.

Rainsy said on Monday it was legitimate to seek to topple Hun Sen because the CNRP had been dissolved and Hun Sen was not prepared to hold free and fair elections in the future.

"Popular uprising is the only option left to Cambodian democrats to bring about a democratic change," Rainsy told Reuters by email.

Rainsy has said he will return to Cambodia on November 9, four years after fleeing to France following a conviction for criminal defamation in which he was ordered to pay $US1 million in compensation.

He also faces a five-year prison sentence in a separate case.

Rainsy said he would reconsider his return and his call for a popular uprising if Hun Sen released Sokha, reinstated the CNRP and organised a genuine election in the future.

Sokha has been detained for two years, the past year under house arrest, while awaiting trial on charges of plotting to overthrow the government.

Australian Associated Press