A criminal investigation has now been opened against him "for participation in an extremist group", the Tajik State Committee for national security told AFP in a statement.

"The initial investigation, based on witness testimony and evidence, confirms Usmonov's membership in Hizb ut-Tahrir," it said.

It added that Usmonov allegedly had long maintained contact with Hizb ut-Tahrir figures and distributed its materials with the aim of bringing about a "violent seizure of power and change in the constitutional make-up of Tajikistan."

The BBC, which gives his age as 59, has said it "has no reason to believe these allegations" and also noted allegations by Usmonov's family that he appeared to have been beaten up by the Tajik security services.

The British embassy has urged Dushanbe to clarify the situation while the OSCE has said that his arrest "raises concerns about undue limitations on free expression."

Usmonov has worked for the last decade for the BBC's Uzbek-language operation. Turkish-speaking Uzbeks form the largest minority in Tajikistan, where the national language is derived from Persian.

Hizb ut-Tahrir was founded in the 1950s in the Middle East and advocates the establishment of a Islamic "caliphate" across Muslim Central Asia, although its members insist they believe this should be achieved by peaceful means.

It appeared in Central Asia around a decade ago and its attempts to recruit new members and distribution of anti-government literature has worried the authorities, particularly in Tajikistan, the poorest state in the ex-USSR.

In the past decade, Tajikistan has convicted some 500 men and womenfor membership of Hizb ut-Tahrir. This year alone, 40 people convicted of membership of the group have been jailed.

Copyright APP (Associated Press of Pakistan), 2011

Copyright AFP (Agence France-Presse), 2011