Cambodia is currently chair of ASEAN, which late last year sprung a surprise by barring Myanmar's junta from key meetings over a failure to honour a peace plan agreed with the bloc.

Incumbent Prime Minister Hun Sen is widely expected to win the election after the Supreme Court dissolved the main opposition Cambodia National Rescue Party last year, leaving no significant competitor for Sen's Cambodian People's Party.

The long-serving PM has been regularly accusing outlets critical of his government of spreading fake news.

"The January 7 victory saved the lives of people who survived the killings and brought back to the Cambodian people rights lost under the regime of Pol Pot," Hun Sen said at the ceremony.

Kem Sokha, head of the banned opposition Cambodia National Rescue Party, was arrested in September.

This is the strongest threat of action from representatives of any Western donors since the arrest of opposition leader Kem Sokha in September.

Hun Sen's government has asked the Supreme Court to dissolve CNRP after its leader, Kem Sokha, was arrested on September 3 and charged with treason.

The latest move comes amid growing concern from the UN and rights groups over what they say is Cambodia's deteriorating human rights and political situation.

Ahead of a crucial court ruling, the opposition demanded "immediate and unconditional release" of their leader, who faces charges of treason.

The Kem Sokha led CNRP remains undeterred in the face of the ban threat by PM Hun Sen, confirming its participation in the 2018 national election.

PM Hun Sen has threatened dissolution of the CNRP after its members boycotted a parliamentary vote to prosecute Kem Sokha, their leader, charged with treason.

The arrest marks an escalation in a campaign against critics, independent media and any potential threats to Hun Sen's hold on power ahead of an election next year at which Kem Sokha has been expected to be his main challenger.

The amendment bans former opposition leader Sam Rainsy, who lives in exile in France to avoid arrest in a number of convictions, from campaigning.

Kem Ley, 46, was shot in Phnom Penh in broad daylight last year and his death drew tens of thousands of protesters onto the streets.

Political tension is growing between Hun Sen and the opposition Cambodia National Rescue Party, which wants to take on the prime minister in local elections in 2017 and a general election in 2018.