Cambodia's parliament amended a law on Monday to stop anyone convicted of an offense from running for office, effectively barring long-serving Prime Minister Hun Sen's main rival.
Critics said that the changes approved would undermine the multi-party democracy established in the Southeast Asian state by 1991 peace accords and could turn Cambodia into a de facto one-party state.
Opponents accuse Hun Sen, a former Khmer Rouge guerrilla, of unfair maneuvering to try to retain 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.
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 Association of Southeast Asian Nations Parliamentarians for Human Rights group called it the "death knell for democracy" in Cambodia.
New York-based advocacy group Human Rights Watch said it marked the consolidation of absolute power.
"Cambodia will become a sham democracy going forward," said Phil Robertson, the group's Deputy Asia Director. "The silence of foreign governments and aid donors to this move has been profoundly disheartening."
Cambodia has been transformed during Hun Sen's rule from what was essentially a failed state after decades of conflict.