Russia mourns Moscow concert hall attack victims as death toll rises to 137

People gather at a makeshift memorial for the victims of a shooting attack set up outside the Crocus City Hall concert venue in the Moscow region of Russia [Maxim Shemetov/Reuters]

Russia has lowered flags to half-mast for a day of mourning after scores of people were gunned down with automatic weapons at a rock concert outside Moscow in the deadliest attack inside Russia for two decades.

President Vladimir Putin declared a national day of mourning for Sunday after pledging to track down and punish all those behind the attack, in which at least 137 people were killed, including three children, and more than 150 injured.