The statement from the Investigative Committee gave few details, but said those arrested came from Central Asian countries that once were part of the Soviet Union.
Officials have said the suicide attacker behind Monday's blast that killed 14 people in St Petersburg was a native of the Central Asian country of Kyrgyzstan.
There was no immediate information tying those arrested with the suspected bomber, identified as 22-year-old Akbardzhon Dhzalilov. Fifty-five people wounded in the blast remain hospitalised, deputy mayor Anna Mityanina said today.
Meeting with the heads of security services from a regional alliance that includes most of Russia's Central Asian neighbours, President Vladimir Putin warned that terrorism remained a threat to all in the region.
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"We see that, unfortunately, the situation is not improving," he said. "The recent tragic events in St. Petersburg are the best confirmation of this. We know that each of our countries, practically every one, is a possible and potential target of terrorist attacks."
Earlier today, the Investigative Committee said in a statement they searched the home of Dhzalilov in St Petersburg. They also examined CCTV footage from outside Dzhalilov's home which shows him leave home with a bag and a backpack.
After the attack, several Russian politicians have called for ending the moratorium on capital punishment. But parliament speaker Vyacheslav Volodin dismissed the statements as attention-getting devices and upbraided the politicians, saying "One must not use a tragedy to promote oneself.