PHNOM PENH (Reuters) - Cambodia began registering political parties on Monday for a July general election likely to extend the 33-year-old rule of Prime Minister Hun Sen after the Supreme Court dissolved the main opposition party.

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The National Election Committee (NEC) said Hun Sen’s Cambodian People’s Party (CPP) was the first to register along with the little known, pro-government Cambodian Youth Party (CYP). Registration will end on May 14.

“Many political parties will register - 16 parties have collected forms to fill in,” NEC spokesman Dim Sovannarom told Reuters.

The registration comes amid a campaign by Hun Sen against his critics, including the opposition Cambodia National Rescue Party. The CNRP was dissolved and its lawmakers banned from politics in November after the Supreme Court ruled that it had tried to overthrow the government with U.S. help.

Both the CNRP and the United States deny the accusation.

The CNRP, which had drawn the support of a new generation of voters not content with what they see as the corruption and nepotism that has stalked the Southeast Asian nation’s politics.

The dissolution of the CNRP was followed by the arrest of its leader, Kem Sokha.

Rhona Smith, a U.N. human rights expert on Cambodia, said Hun Sen’s party has “one final opportunity to reverse the current trajectory” and called for the immediate release of opposition leaders and removal of a ban on the opposition.

“No election can be genuine if the main opposition party is barred from taking part,” Smith said in a statement on Monday. “A liberal multi-party democracy is an essential, entrenched and non-amendable feature of the Constitution of Cambodia.”

Hun Sen defended the election, saying it would involve multiple political parties with different viewpoints and take place as planned on July 29. “The process of multi-party democracy continues,” Hun Sen told students at a university graduation ceremony on Monday.

Mu Sochua, a deputy leader of the CNRP, said the election would not be fair. “No CNRP, no free and fair election, means that the next government will be illegitimate,” Mu Sochua told Reuters.