The blast happened at the end of the evening rush hour
At least 10 people have been killed in an explosion outside an underground railway station in Moscow.
More than 50 others were hurt in the blast outside the Rizhskaya station in the north of the city.
Russian officials are blaming the attack on a female suicide bomber, seen outside the station before the blast.
An Islamist group has issued a statement claiming responsibility for the bombing and warning it would carry out further attacks.
The statement, said to be from a group calling itself the Islambouli Brigades, described the attack as "part of the wave of support and assistance to the Muslim Chechens".
The statement, posted on a website used by militants in the past, said the attack would be "followed, with God's help, by more waves until we humiliate the state of heresy called Russia".
The same group claimed responsibility for attacking two Russian planes that crashed within minutes of each other last week, killing 89 people.
Witnesses at the scene of Tuesday's bombing described seeing a woman suspect heading for the metro.
When she spotted police officers at the entrance, witnesses say the woman turned back and detonated a bomb in the street.
Police are still exploring a second theory that a device may have been planted beneath a car.
Moscow's mayor has spoken of at least 1kg (2.2lbs) of explosives, packed with ball bearings and metal bolts.
The station's windows and those in nearby shopping centre were blown out by the force of the blast, which happened soon after 2000 (1600 GMT).
More than a dozen people could be seen lying on the pavement and grass surrounded by pools of blood.
The underground station is on one of the busiest routes to the north of the city, says the BBC's Damian Grammaticas in Moscow.
"There are children among the wounded," a police source was quoted as saying.
Sergei Pyslaru, who was driving on a nearby street at the time of the explosion, told the Associated Press news agency that he heard a powerful blast followed by a smaller one.
"I thought my roof would come off," he said.
Alexei Borodin was out walking with his mother when he heard a powerful bang.
"Something flew past my head, I don't know what it was," he told AP.
"There were people lying in the square. There were pieces of bodies...We were walking through pieces of people. One young guy tried to get up and couldn't," he said.
Earlier, Russian President Vladimir Putin said the bombs that brought down two passenger planes last week proved the close links between what he called "destructive forces'' in Chechnya and international terrorism.
"If a terrorist organisation has taken responsibility for this and that group has links to al-Qaeda, then that confirms the ties between certain forces active in Chechnya and international terrorism," he said.
However, Mr Putin said the claim itself had not been verified.