At least 38 people have been killed after two female suicide bombers blew themselves up on Moscow Metro trains in the morning rush hour, officials say.
Twenty-four 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 14 dead.
The FSB said it was likely a group from the North Caucasus was responsible.
The BBC's Richard Galpin in Moscow says no group has yet said it carried out the attacks, but past suicide bombings in the capital have been carried out by or blamed on Islamist rebels fighting for independence in Chechnya.
In February, Chechen rebel leader Doku Umarov said "the zone of military operations will be extended to the territory of Russia... the war is coming to their cities".
At an emergency meeting with senior officials, President Dmitry Medvedev vowed to uphold the "policy of suppressing terror and the fight against terrorism".
"We will continue operations against terrorists without compromises and to the end," he said.
Federal security forces have scored a series of successes against militants in the North Caucasus in recent weeks. In February, at least 20 insurgents were killed in an operation by troops in Ingushetia.
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, lies beneath the headquarters of the Federal Security Service (FSB).
"I was moving up on the escalator when I heard a loud bang, a blast. A door near the passage way arched, was ripped out and a cloud of dust came down on the escalator," an eyewitness named Alexei told Rossiya 24 TV channel.
"People started running, panicking, falling on each other," he said.
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.
"I was in the middle of the train when somewhere in the first or second carriage there was a loud blast. I felt the vibrations reverberate through my body," one passenger told the RIA news agency.
"People were yelling like hell," he said. "There was a lot of smoke and within about two minutes everything was covered in smoke."
More than 60 people were injured in the two attacks, 30 of them badly, officials said.
In a meeting with President Medvedev, FSB chief Alexander Bortnikov said its investigators believed the attacks had been carried out by "terrorist groups related to the North Caucasus".