The European Union and rights groups say the Cambodian government's move to dissolve the main opposition party would be a serious blow to democracy and undermine the credibility of elections next year.
Human Rights Watch said Tuesday that the government of long-serving Prime Minister Hun Sen is making a "naked grab for total power."
Cambodia's government took initial legal steps last week to dissolve the Cambodia National Rescue Party, its latest move to kneecap the opposition ahead of next year's general election.
Two minor political parties believed to be operating on behalf of the government asked the Supreme Court to disband the opposition on the ground that it was allegedly involved in a plot to topple Hun Sen's administration.
Human Rights Watch along with other rights groups and the European Union are calling on the government to drop the court case.
"Governments that still insist Cambodia is democratically ruled should act to reverse this development, or share the blame for democracy's demise under Hun Sen's autocratic rule," Human Rights Watch said. An EU statement said the situation is "very worrying" and urged the government to reconsider the legal steps.
The CNRP is the only party aside from the ruling Cambodian People's Party with representatives in parliament, and no third party comes close in terms of popularity and support. The CNRP posed an unexpectedly strong challenge in 2013's general election and the government has since taken steps to tighten its grip on power.
The CNRP has denied seeking to unlawfully or violently overthrow the government, saying the charge is politically motivated.
If the Supreme Court finds the opposition party guilty of violating the Political Party Law, not only would it be dissolved, but its leaders would be banned from involvement in politics for five years.
The court is likely to uphold the complaint, since the country's judiciary is widely considered to operate at the behest of Hun Sen.
Son Chhay, a senior opposition politician, said millions of Cambodians support the CNRP and the ruling party should resume dialogue with it rather than taking steps that will damage Cambodia's reputation.
In other recent moves that tightened the grip of Hun Sen's government, an independent English-language newspaper, The Cambodia Daily, was shut down after being accused of not paying a huge tax bill, an assessment it strongly disputed.
More than a dozen radio stations that broadcast dissident voices or used programming from the U.S. government-funded Voice of America and Radio Free Asia were forced to stop broadcasting for alleged breach of regulations.
