PHNOM PENH (Reuters) - Cambodia’s parliament amended a law on Monday to stop anyone convicted of an offence from running for office, effectively barring long-serving Prime Minister Hun Sen’s main rival.

Cambodia's Prime Minister Hun Sen (L) signs a register as he arrives before a plenary session at the National Assembly of Cambodia, in central Phnom Penh, February 20, 2017. REUTERS/Samrang Pring

The U.S. embassy said it was deeply concerned about that and the other changes adopted. Some critics said they were a step to turning the Southeast Asian country of 16 million into a de facto one-party state.

Opponents accuse Hun Sen, a former Khmer Rouge guerrilla, of unfair maneuvering to try to keep his three-decade grip on power at local elections in June and a general election next year.

The ruling Cambodian People’s Party voted to change the 1998 election law to ban parties that engage in activities that include incitement, promoting secession or anything that could harm national security.

Politicians convicted by a court are banned from standing for election and their parties can be dissolved.

That would exclude veteran opposition leader Sam Rainsy, who has been convicted of a series of defamation charges and has lived in exile in France since 2015 to avoid them.

He resigned from the Cambodia National Rescue Party (CNRP) this month, saying he wanted to save his party in the face of the potential ban. He rejects the charges against him as politically motivated.

In comments emailed to Reuters, Rainsy said the passage of the bill marked one of Cambodia’s darkest days since 1991 peace accords, which drew a line under decades of conflict that had left Cambodia a failed state.

“The international community must address the fact that they have funded a democratic system which is now lurching dangerously towards a one-party state,” he said.

The CNRP’s 55 lawmakers boycotted the National Assembly vote on Monday, saying it had targeted them. But Hun Sen’s party has a slim majority in parliament, so it was able to pass the change.

Welcoming the change to the election rules, ruling party lawmaker Chheang Vun said it would allow the interior ministry to start closing some of Cambodia’s 76 political parties. He said only 45 were properly registered.

The U.S. embassy in Phnom Penh said it was concerned at amendments passed with little consultation or public debate and called on the government to ensure fair elections.

“The amendments give the government broad authority to restrict freedom of expression and the legitimate activities of political parties and, under vaguely defined circumstances, to dissolve them,” it said.

The Association of Southeast Asian Nations Parliamentarians for Human Rights group called the measure the “death knell for democracy” in Cambodia. New York-based advocacy group Human Rights Watch said it marked the consolidation of absolute power.