At least 36 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 12 dead. The FSB said it was likely a group from the North Caucasus was responsible.
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.
Outside Lubyanka Station, traffic has been restored, but Lubyanka Square remains closed to pedestrians - there are only police, officials and journalists. 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 70 people were injured in the two attacks, 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". "Fragments of the bodies of two female suicide bombers were found earlier at the scene of the incident. According to preliminary evaluations, these individuals were, by place of residence, linked to the North Caucasus," he said.
Citing a preliminary forensic report, Mr Bortnikov said that the devices had been made with the powerful explosive, hexogen, which is more commonly known as RDX.
The bomb that went off at Lubyanka station had an equivalent force of up to 4kg of TNT, while the bomb at Park Kultury was equivalent to 1.5-2kg of TNT, he said. The devices were filled with bolts and cut-up metal bars for shrapnel. Federal prosecutors said they had opened an investigation into "suspected acts of terrorism".
Parts of the Metro system have been closed down as a precaution, and 700 interior ministry troops have been deployed on the streets.
"The whole city is a mess, people are calling each other, the operators can't cope with such a huge number of calls at a time," said Olga, a BBC News website reader in Moscow. "Those who witnessed the tragedy can't get over the shock."
President Medvedev asked officials to increase security on the public transport system nationwide. "What was being done needs to be substantially strengthened," he said.
"[We must] look at this problem on the scale of the state, not only as it applies to a particular type of transport and a particular city."
Prime Minister Vladimir Putin, who is visiting the Siberian city of Krasnoyarsk, said a crime that was "terrible in its consequences and heinous in its manner" had been committed. "I am confident that law enforcement bodies will spare no effort to track down and punish the criminals. Terrorists will be destroyed," he added.