Up to 40 people have been reported killed in twin explosions at stations on Moscow's metro rail network, Al Jazeera reported.
The first blast took place at the Lubyanka metro station in the centre of the city, followed by a second explosion at Park Kultury in the southwest of the city.
The cause of the explosions was not immediately clear, but Russian prosectuors said they had opened an investigation into a possible act of "terrorism".
Speaking about the Lubyanka attack, Irina Andrianova, a spokewsoman for Russia's emergencies ministry, told the Itar-Tass news agency: "14 people died in the wagon of the train and 11 on the platform.
"The blast hit the second carriage of a metro train that stopped at Lubyanka, at 07:56 (03:56 GMT)."
The headquarters of Russia's Federal Security Service (FSB), successor to the Soviet-era KGB, is located just above Lubyanka station.
At least 15 people were reported to have been killed in the later blast at Park Kultury.
"Both attacks took place on what is known as the Red Line, or line number one," Al Jazeera's Neave Barker, reporting from Moscow, said.
"This has to be seen as a politically motivated attack targeting what is perhaps one of the most symbolic signs of Russian authority in the capital when thousands of people are using one of the most heavily used metros in the world."