Moscow officials say the explosion struck between two stations that lie at the heart of the underground commuter train lines in this city of some 10 million, where most people take public transportation to work.

The explosion set off a severe fire in the metro tunnels deep below the ground and filled the tunnels with heavy black smoke. Rescue workers have since evacuated hundreds of people to safety. Many of those rescued are now being treated for burns and smoke inhalation.

More than 100 buses have been dispatched to the area to transport huge crowds of passengers stranded in a wet snowfall.

A spokeswoman for Russia's interior ministry, which is investigating the blast along with Russia's Federal Security Service, says terrorism is the most likely cause of the explosion. But she says investigators are also looking at other possible causes.

Russia's Itar-Tass news agency quotes a federal investigator as saying the explosives used in the blast had the power of more than one kilogram of TNT. The official adds that the impact of the blast would have been multiplied several times in the closed space of a tunnel.

The Russian capital has been on alert, and security measures reportedly increased, ever since a series of suicide bombings last year blamed on Chechen separatist rebels. The last such attack was in December, when a female suicide bomber blew herself up just opposite the Kremlin, killing five people.

Chechen separatists are also suspected in an explosion earlier this week in the southern Russian city of Vladikavkaz, near the Chechen border, which killed two people and injured 10 others.

Suicide bomb attacks have become a grim hallmark of the Chechen rebels, who have been locked in a bloody guerrilla-style war with Russian federal forces for most of the last decade.