The explosion at a bus stop in central Moscow on Monday night was caused by a hand grenade, according to the police, the Interfax news agency reported Tuesday.

“Police discovered than an F1 grenade exploded Monday night near a bus stop on Pokrovka Ulitsa,” police spokesman Andrei Galiakberov said.

Galiakberov said earlier that the explosion was caused by an "improvised explosive device thrown out of a passing car or from the window of a high-rise building."

The explosion at Pokrovka left five people injured and three of them remain in hospital.

A criminal investigation has been opened into the incident.

There have been no official statements classifying the incident as terrorism, it is being treated as an act of hooliganism.