According to the BBC live coverage, Moscow has suffered a terrible suicide-bomber attack on its subway system similar to the July 7th attacks in London in 2005:
Two bombs at the height of rush hour on Moscow’s Metro system have killed at least 37 people. Russian officials say female suicide bombers set off the blasts within an hour of each other. More than 20 people died in the first bombing, as a train was standing at Lubyanka station, beneath the headquarters of Russia’s main intelligence agency, the FSB. The second bomb was on a train at Park Kultury station.
It is unclear who is responsible at this time, but the New York Times notes:
The attacks marked the second major upsurge in terrorism on the transportation system over the last year. In November, a bomb derailed a luxury train traveling from Moscow to St. Petersburg, killing 26 people. The authorities linked the attack to Muslim insurgents in the Ingushetia region, which is near Chechnya.
The Telegraph suggests that the culprits may be:
Islamist rebels seeking to establish an Islamic caliphate on Russia’s southern tip [who] have largely confined their attacks to the North Caucasus area they want to control in recent years. But a bombing of a passenger train between Moscow and St. Petersburg last November that left dozens dead suggested they may be preparing to widen their campaign to Russia’s big cities.