By Guy De Launey
BBC, Phnom Penh
One of Cambodia's most prominent human rights activists has been arrested and charged with criminal defamation.
The centre was surrounded by police by mid-morning
Kem Sokha, as leader of the Cambodian Center for Human Rights, has been a prominent critic of the government.
A number of opposition figures have been charged with defamation - which can be a criminal offence under Cambodian law - in recent weeks.
Foreign diplomats have expressed concern that charges are being used to silence and intimidate the opposition.
The government has defended its right to prosecute people who it says have broken the law.
Police surrounded the headquarters of the Cambodian Center for Human Rights in the middle of the morning.
They held warrants to search the building and to arrest the organisation's leader.
Foreign diplomats and the United Nations' Human Rights representative tried to intervene but police barred them from entering.
Eventually, Kem Sokha emerged and police took him to court.
Opposition leader Sam Rainsy was sentenced in absentia last week
The defamation charge stems from a banner displayed at a rally to mark International Human Rights Day earlier this month.
One of the organisers of the rally also faces charges.
It means that four critics of the government are now in jail awaiting trial.
Five more have left the country rather than face imprisonment.
Last week, the opposition leader, Sam Rainsy, was sentenced in absentia to 18 months in prison for defaming the leaders of the governing coalition.