Russia detains eight people over Saint Petersburg metro bombing

"Six people were detained in Saint Petersburg and two in Moscow who are involved in the act of terror" following an investigation and search operation conducted with the FSB security service and interior ministry, Russia's powerful Investigative Committee said in a statement.

During their searches investigators discovered an explosive device "identical" to the one found at a Saint Petersburg metro station that was discovered shortly before the deadly blast in a metro tunnel.

Firearms and ammunition were also found at the suspects' residences, the committee said.

Investigators said earlier on Thursday that they had searched the flat of "several citizens of Central Asian republics, who had been in contact" with suspected bomber Akbarjon Djalilov, thought to be a Russian national born in Central Asian Kyrgyzstan.

The Investigative Committee said objects "relevant to the investigation" had been confiscated but did not disclose their nature.

As the probe into the bombing gained pace, thousands of people gathered at officially sanctioned rallies in Moscow and Saint Petersburg to remember the victims.

People clutching flowers and white balloons gathered at a rally outside Saint Petersburg's Technological Institute metro station, one of the two stations at the heart of the attack.