At least 39 people died and more than 100 were injured in a suspected bomb attack on a packed Moscow subway train.
Scores of people were injured in the blast
The blast happened at the height of rush hour in the second carriage of the train as it approached Paveletskaya station from the south.
Hundreds of passengers were evacuated from the station.
President Vladimir Putin blamed the blast on Chechen rebel leader Aslan Maskhadov and called for greater efforts to fight terrorism.
"We do not need any indirect confirmation. We know for
certain that Maskhadov and his bandits are linked to this
terrorism," the Russian Interfax news agency quoted the president as saying.
The Chechen rebel leadership has issued a statement condemning the explosion, although it said it fears Chechen separatists will be blamed.
Russian police and security officials said the blast could have been caused by a suicide bomb attack.
However, Moscow's deputy mayor told the Associated Press news agency that investigators had not found metal shrapnel, which usually fills suicide bombers' explosives.
He said that the bomb was probably hidden in a suitcase or rucksack on the floor of the subway car.
Up to 120 people were injured in the incident, Interfax reported, many of them suffering from broken bones, smoke inhalation and burns.
Speaking at the Kremlin following talks with Azeri President Ilham Aliyev, Russian President Vladimir Putin described terrorism as "the plague of the 21st Century" which the world must unite to defeat.
There were scenes of panic and confusion as people fled the powerful explosion, which occurred at about 0840 (0540 GMT).
Helicopters from the Russian Emergencies Ministry hovered over the Paveletskaya station as police with loudspeakers told the gathered crowd that the metro was closed.
Scores of dazed commuters, many with faces bloodied by injuries and blackened with smoke, poured into the city streets to escape the flames.
One woman who works in a shop near the site described horrific scenes as emergency workers struggled to cope with the scores of victims.
"We saw them carrying bodies and injured covered in blood," she told French news agency AFP.
"A man came into the shop, he was shaking uncontrollably and covered in blood.
"He told us that he saw arms, legs scattered around the carriage. He said it was bloody carnage."
The train had left Avtozavodskaya station, on the metro system's green line, and was travelling north towards the city centre to Paveletskaya station when the blast occurred.
Wounded passengers were guided to safety through the tunnel by emergency services, while fire officials attempted to put out a large fire caused by the explosion.
Moscow's deadliest attacks
Dec 2003 - female suicide bomber kills five near Red Square
July 2003 - aborted suicide attack kills bomb disposal expert
July 2003 - suicide bombers kill 14 at Moscow rock concert
Oct 2002 - suicide attackers seize Moscow theatre, 130 hostages die in rescue
Aug 2000 - bomb in underpass kills 11
Sept 1999 - two blasts in blocks of flats kill more than 200
More than 50 ambulances arrived at the scene and police sealed off the surrounding streets.
The train was severely damaged in the blast, which occurred as many commuters were travelling to local offices and factories.
BBC correspondent Steve Rosenberg in Moscow says that more than nine million people use the metro system every day and the blast is likely to spread fear across the city.
Chechen militants have previously targeted the Russian capital with suicide missions and the city has been on alert for attacks as the country prepares for presidential elections in March.