Russian police have detained 271 people, most of them from the North Caucasus and central Asia, in an investigation into involvement in "terrorist activities", authorities in St Petersburg said on Saturday.
Russia is concerned that Islamist militants could become a greater threat outside the heavily Muslim North Caucasus region, plagued by an insurgency rooted in two post-Soviet separatist wars in the republic of Chechnya.
In a statement, the regional investigative committee in St Petersburg said that most detainees were from the North Caucasus and the former Soviet republics of Uzbekistan, Tajikistan and Azerbaijan. An Egyptian and an Afghan were also detained.
The committee said they were detained "in order to check if they had legal grounds for being in St Petersburg and their possible involvement in terrorist activities."
They were detained during an overnight raid on St Petersburg's oldest market.
Authorities said security forces had been searching for extremist literature, weapons, drugs and documents related to a recently-launched criminal case in connection with "public justification of terrorism and incitement of hatred".
The authorities did not say whether any of those detained were suspected of involvement in plotting or carrying out attacks.
Many market traders in Russian cities are from the North Caucasus or central Asia.
Local media said police had initially detained 700 people.