Hizb ut-Tahrir (Party of Liberation) seeks to re-establish a caliphate -- a pan-Islamic state based on Islamic rule harking to the mediaeval era -- and has been banned in Russia since 2003.
"People linked to the organisation of the activities of a Saint Petersburg cell of the international terrorist organisation Hizb ut-Tahrir were detained," a statement from the federal security service (FSB) said.
The security agency did not specify the number of those held but local news reports said some 20 people were detained following a string of raids on apartments and hostels across the city today morning.
In Russia, the group has been active in the Northern Caucasus as well as in the mainly Muslim Volga regions of Bashkortostan and Tatarstan.