The first blast occurred outside an imambargah (Shia prayer hall) in Abbas Town at about 7 P.M. As people were leaving after prayers. The second explosion went off within 10 minutes, local residents said.
Rescue officials said 48 people were killed in the blasts which caused extensive damage to residential buildings and destroyed several shops in the area.
Sindh province Health Secretary Suresh Kumar put the toll at 45. No group claimed responsibility for the attack.
The toll is expected to increase as some of those injured are critical. "The blasts have caused a crater four feet deep near the main entrance," Senior Superintendent of Police Mazhar Maswani said.
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An emergency was declared at hospitals across Karachi and doctors performed emergency surgeries to save victims.
Several young children were among the seriously injured, officials said.
Police officials said the first blast was caused by explosives hidden in a car that was parked near the entrance of Abbas Town.
There were reports that the second blast was caused by the gas cylinder of a car, though this could not immediately be confirmed. The blasts caused a power outage and triggered panic in Abbas Town, which has a large Shia population.
The first explosion destroyed the fa�ade of two apartment blocks and damaged about 20 shops and several cars.