Cambodian authorities on Wednesday refused permission for the 18 opposition-aligned trade unions and associations to celebrate an International Labor Day event on May 1 at the capital's Freedom Park.

"The Phnom Penh Municipality does not allow the trade unions and associations to organize this event at Freedom Park (the capital's designated protest space) or other public places because the Freedom Park is still under the court's investigation process over violent cases in January," said a letter signed by Phnom Penh Vice-Governor Khuong Sreng and sent to those trade unions and associations.

The letter stated that those unions and associations can mark May Day at their offices, factories, or private places without marching.

Pav Sina, president of the Collective Union of Movement of Workers, said Thursday that the 18 opposition-aligned trade unions and associations still planned to rally at the park.

"We expect that a large number of workers will join us. If the authorities do not allow us to enter the park, we will meet and march outside the park," he said.

He said the purpose is to demand the monthly minimum wages of 160 US dollars for garment workers and to call for the release of 23 protesters still detained during violent clashes in January.

On Wednesday, security has been tightened at the Freedom Park as police have deployed barbed wire barricades around the park to prevent people from entering the site.

Rallies and protests, organized by the country's opposition Cambodia National Rescue Party and its aligned trade unions, have often occurred since a national election in July last year gave victory to Prime Minister Hun Sen's ruling party.

The opposition has refused to accept the election results, which it says were marred by fraud, and has boycotted parliament and staged a number of demonstrations to demand an early election.