An explosion struck a metro station in Russia’s second-largest city St. Petersburg, leaving several people injured, a local law enforcement source told TASS on Monday.

“A blast occurred at Sennaya Ploshchad metro station, several people have been injured,” the source said, adding that the station has been overtaken by smoke.

Evacuation is under way at a blast-hit subway station. Seven metro stations in the city have been closed following the blast:

“Park Pobedy, Elektrosila, Moskovskiye Vorota, Frunzenskaya, Tekhnologichesky Institut, Sennaya Ploshchad and Gostiny Dvor metro stations have been closed,” the metro’s press service reports.

A spokesman for the regional department of Russia’s Emergencies Ministry said it was checking reports on the smoke in the metro. “There is a complaint about the smoke, the staff members are going to the scene,” he said.