Twenty one people including three children died and 90 others were injured in a powerful bomb blast targeting a prayer hall of the minority Shias in the northwestern Pakistani city Peshawar on Friday.The blast occurred outside an Imambargah on a busy street in Kucha Risaldar area of the walled city.Hospital sources said at least 21 people died in the blast which occurred in the evening hours when Shia shoppers crowded the area ahead of the Eid-ul-Azha festival.About 25-30 kg of explosives were used in the huge blast, which caused extensive damage to the prayer hall and some nearby houses, police said. It triggered a massive fire in shops and warehouses in the area and also caused a power outage, hampering rescue work in the congested locality.The explosives were believed to have been hidden inside a vehicle. The blast caused a five-foot crater in the area, police officials said.Rescuers put the number of the injured at 90. Ambulances rushed to the area and took the injured to nearby hospitals where emergency has been declared. Reports said three children and four women were among the dead.No group has claimed the responsibility for the attack.Earlier also there have been several instances of targeting Shia prayer halls in Peshawar and other parts of the North West Frontier Province in the past few months.