At least 35 people have been killed after two female suicide bombers blew themselves up on Moscow Metro trains in the morning rush hour, officials say.Twenty-three died in the first blast at 0756 (0356 GMT) as a train stood at the central Lubyanka station, beneath the offices of the FSB intelligence agency.
About 40 minutes later, a second explosion ripped through a train at Park Kultury, leaving another 12 dead.
No-one has said they carried out the worst attack in the capital since 2004.
But the BBC's Richard Galpin in the Russian capital says past suicide bombings there have been blamed on Islamist rebels fighting for independence in the troubled North Caucasus region of Chechnya.
In February, Chechen rebel president Doku Umarov warned that "the zone of military operations will be extended to the territory of Russia... the war is coming to their cities".
Moscow's metro is one of the busiest subways in the world, carrying some 5.5m passengers a day.
'No fire'
Emergency services ministry spokeswoman Irina Andrianova said the first explosion tore through the second carriage of a train as it stood at Lubyanka at the peak of the rush hour.
The station, on both the busy Sokolnicheskaya and Tagansko-Krasnopresnenskaya lines, is close to the headquarters of the Russian Federal Security Service (FSB).
The BBC Russian Service's Yuri Maloverian at Lubyanka station says there are still ambulances on the scene although all the wounded have been transported to local hospitals.
The second blast at Park Kultury, which is six stops away from Lubyanka on the Sokolnicheskaya line, came at 0838 (0438 GMT). It struck at the back of the train as people were getting on board.
"According to preliminary information provided by the Federal Security Service, two female suicide bombers were involved," Moscow Mayor Yuri Luzhkov told reporters gathered outside Park Kultury.
In the Park Kultury blast, the bomber was wearing a belt packed with plastic explosives and set it off as the train's doors opened, a spokesman for Russia's top investigative body said.
Federal prosecutors said they had opened an investigation into "suspected acts of terrorism".
The entire Metro system has been closed down as a precaution, while the Russian civil aviation regulator has ordered all local airports to increase security.
Prime Minister Vladimir Putin, who is currently visiting Siberia, is "receiving detailed information from security agencies and social services about the work on helping the victims", a spokesman said.
Russian forces have scored a series of successes against militants in recent weeks. In February, at least 20 insurgents were reportedly killed in an operation by Russian security forces in Ingushetia.
There was a major attack on the Moscow Metro in February 2004, when at least 39 people were killed by a bomb on a packed train as it approached the Paveletskaya Metro station.
Six months later, a suicide bomber blew herself up outside a station, killing 10 people. Both attacks were blamed on Chechen rebels.
In November, the Caucasian Mujahadeen claimed responsibility for a bomb that killed 26 people on board an express train travelling from Moscow to Russia's second city of St Petersburg.
