The Senate today passed controversial amendments to the Constitution and Penal Code despite an appeal from the United Nations to reconsider their impact on free speech and political participation.

The amendments include a ban on insulting the king, which carries a punishment of one to five years in prison and a fine of 2 million riel to 10 million riel (about $500 to $2,500). Similar laws have been used to curb free speech and crackdown on political dissent in Thailand.

The laws will be sent to King Norodom Sihamoni for final approval.

Among the Constitutional amendments is a requirement that political parties “place the country and nation’s interests first”.

Previously approved by the Council of Ministers and National Assembly, the amendments have been roundly condemned by the UN and rights groups. The passage today was met with fresh condemnation from the Cambodian Centre for Human Rights, Asean Parliamentarians for Human Rights, and more than 60 other organisations. A joint statement warns that the amendments will have "grave implications" for Cambodia and urges that they be rejected.

"The vague nature of the amendments means they could be misused to justify the introduction of wide-ranging sanctions punishing the legitimate exercise of fundamental freedoms," the statments says.

The statement also condemned the "rushed and secretive" nature of the drafting process, which did not include any input from the public or civil society.

"These amendments would provide yet more legal weaponry to a government that appears determined to eliminate all forms of peaceful dissent," it continues.

Constitutional amendments must be passed with a two-thirds majority in both the National Assembly and the Senate. They were approved unanimously in the National Assembly, where little political opposition remains following the dissolution of the Cambodia National Rescue Party.

The 11 senators from the Candelight Party, the remnants of the former Sam Rainsy Party and the only remaining vestige of the opposition in Cambodia, boycotted today's vote.

However, the ruling Cambodian People's Party holds 46 seats in the 61-seat body, and the amendments easily passed this morning with 45 votes.