PHNOM PENH, Cambodia - Police in Cambodia detained eight activists, including two Westerners, who wore black clothing in a peaceful protest Monday in support of human rights workers who were jailed last week, a rights advocacy group said. They were released after being held for several hours.

Om Sam Ath of the group Licadho said the eight were among as many as 200 people taking part in a "Black Monday" campaign to show their solidarity with one former and four current officers of another human rights group, ADHOC. He said the two Westerners were a German and a Swede who worked as advisers to Licadho.

The detainees were released without immediately facing charges. National Police spokesman Gen. Kirt Chantharith said the six Cambodians were detained because they sought to incite disorder, but were released after they agreed to sign letters recognizing their mistake.

The Westerners were also released, but police kept their passports, Om Sam Ath said.

The five people jailed last week are accused of conspiring to bribe a woman to deny that she had an affair with the deputy leader Kem Sokha of the opposition Cambodia National Rescue Party. The arrests were widely regarded as the latest in a series by Prime Minister Hun Sen's government putting legal pressure on its critics and political opponents.

The government had warned that the protest would be considered an illegal act of rebellion, describing the intended wearing of black as a "colour revolution." At least 100 police blocked the route to Prey Sar prison, where the marchers intended to hold a Buddhist ceremony to pray for the release of those arrested last week.

Om Sam Ath said there had been no violence during the protest.

"These people have done nothing wrong and I don't know why they would be arrested," he said.

Phnom Penh Vice Governor Khuon Sreng told reporters that security forces had been deployed to defend the government from being toppled by a colour revolution group.

"We deployed the forces to protect the safety of the people from the group committing anarchy," he said.

News from © The Associated Press, 2016