There were scenes of panic and carnage near a busy Moscow subway station where police say a female suicide bomber set off explosives.
The blast set two nearby cars on fire as ambulances converged on the scene to help the wounded.
Television stations gave out various phone numbers asking for people to call if they had any more information.
The incident comes exactly a week after two Russian airliners crashed within minutes of each other, killing a total of 90 people.
Authorities say the crashes were likely caused by female suicide bombers from the breakaway region of Chechnya, based on evidence they've gathered.
Russian troops have been fighting separatist rebels in Chechnya for most of the past decade.
And on Sunday there was a controversial election for president in the war-torn republic which brought a Kremlin-backed candidate to power. He replaces the previous president who was assassinated in a bomb attack last May.
An Islamic group calling itself the Islambouli Brigades claimed responsibility for the airline crashes as well as Tuesday's bombing, saying it was acting on behalf of Chechen fighters.
Chechen bombers have struck often in Moscow in recent years.
In February more than 40 people were killed in an explosion on the city's underground metro at the height of morning rush hour.
And two Chechen women detonated explosives at an outdoor rock concert near Moscow last year, killing more than a dozen people.
President Vladimir Putin has firmly ruled out any talks with the Chechens about their bid for independence, saying that the conflict there is part of the global war against international terrorism.