Terrorist attack?

Prosecutors have opened a criminal probe into possible acts of "terrorism" after the deadly blasts. Russian Prosecutor General Yuri Chaika would directly supervise the investigation into the blasts, said a spokeswoman for the Prosecutor General's Office.

The blasts, described by a police source as "a well-planned terrorist attack," were the first terrorist attack on the Russian capital for six years.

Russia's top investigator Vladimir Markin said preliminary investigation showed the blasts were aimed at hitting as many people as possible.

Police with sniffer dogs were carrying out inspections of other stations as the capital remains on high alert. Security has been tightened at the capital's overland train terminals and airports.

Emergencies ministry chief Sergei Shoigu has reported to President Dmitry Medvedev on the ongoing operation.

Russian Prime Minister Vladimir Putin was being informed of the blasts in the Siberian city of Krasnoyarsk, where he was on a visit, the spokesman Dmitry Peskov said.

If terrorism is confirmed as the cause of the blasts, this will be the first major terrorist incident in the Russian capital since autumn 2004, when 90 people died in two plane bombings and 10 people were killed in a bomb attack outside a north Moscow metro station.

Moscow subway system has a daily delivering capacity of over eight million, which is one of the busiest metro systems in the world.