A labor leader received a threatening letter Wednesday, following demonstrations over the weekend by workers upset by the ongoing Preah Vihear standoff.

Chea Mony, who became the president of the Free Trade Union after the murder of his brother, Chea Vichea, in 2004, said he received a threatening letter by e-mail, asking to join the ruling Cambodian People's Party or live in insecurity.

"They threatened me, wanting me to join the CPP and Human Rights Party," Chea Mony said Wednesday.

Chea Mony led a demonstration of several hundred workers on Sunday to protest troop deployments at Preah Vihear temple, but authorities broke up the rally.

He had not filed a complaint with authorities, Chea Mony, said, because he had no faith in their ability to protect him.

Chea Vichea was gunned down in broad daylight, and two men widely believed to be innocent are serving 20-year sentences for the murder.

No suspects have ever been arrested in a number of high-profile murders over the years, including union leaders Ven Som Bunthoeun and Ros Sovannaret, Funcincpec official Oam Rathsady and film star Piseth Pilika.

The murder of an opposition journalist, Khim Sambor, who was shot in July ahead of national elections, has so far gone unsolved.

Ministry of Interior spokesman Khieu Sopheak said it was Chea Mony's right whether to file a complaint, but he said the police have "tried our best" to arrest suspects in the murders.

Chan Saveth, a rights investigator for Adhoc, said the authorities should take the threat seriously.

"If it's true, we are concerned about the safety of Mr. Chea Mony," Chan Saveth said.