Russia's FSB security agency said Friday it had detained seven nationals from Central Asia who were "preparing terrorist attacks" in Saint Petersburg, which suffered a metro bombing in April.
Those held were suspected of planning to hit "the railway system and major public gathering places" in Russia's second city, the FSB said in a statement.
Russia has stepped up security since a suicide bomb attack in the Saint Petersburg subway killed 15 and injured dozens on April 3.
The alleged perpetrator, Akbarjon Djalilov, was a 22-year-old Russian citizen who was born in ex-Soviet Kyrgyzstan in Central Asia.
The bombing was claimed by a little-known group, the Imam Shamil Battalion, which experts say is linked to al-Qaeda.
Jihadists including the Islamic State group have repeatedly threatened to attack Russia over its intervention in Syria on the side of leader Bashar al-Assad.
Citizens of Central Asia have been prominent among foreign fighters who headed to Syria and Iraq to fight for jihadist groups.
Russia hosts large populations of mainly migrant workers from ex-Communist Central Asia and they face frequent harassment and discrimination in the country.
The Russian security services regularly announce that they have thwarted plans for attacks.
The prosecutor's office said Friday that twelve attacks have been foiled on Russian soil since the beginning of the year, state news agency RIA Novosti reported.