At least ten people have been killed and over 20 injured in an explosion in the St. Petersburg metro between the Sennaya Ploshchad and Tekhnologichesky Institut stations, a law enforcement source told Interfax on Monday.

"About ten people were killed and over 20 others injured in an explosion that occurred between the Sennaya Ploshchad and Tekhnologichesky Institut stations in the St. Petersburg metro. The explosive device was equivalent to 200-300 grams of TNT," the source said.

There is still smoke at the said stations, the source said.

Initial investigative actions have been launched.