The blast took place around 7 pm at Abul Hassan Isphahani road, Xinhua reported.
Sachal Ahmad Qamar, deputy inspector general of Karachi Police, said the first explosion targeted a group of people offering evening prayers in a mosque in Iqra City area.
He, however, said the second blast happened due to a gas cylinder explosion in a car which caught fire following the first blast.
Karachi Police official Fayyaz Khan said the first bomb was triggered off by a remote-controlled device. Explosive materials were placed in a mini-truck parked near the entrance gate of Iqra City area.
He said the bomb disposal squad was investigating the nature of the second blast.
According to initial reports, the second explosion was carried out by a suicide bomber who blew himself up in the area.
Fayyaz said an estimated 150 kg of explosives were used in the blasts.
The bomb also lethal chemicals that led to a huge fire after the explosions.
The blast destroyed two multi-storeyed buildings comprising 200 flats. Many people are believed to be trapped under the rubble.
Following the blasts, several vehicles and over 150 shops caught fire.
"So powerful was the explosion that the facades of several apartments facing the site of blast were blown off the buildings. Windowpanes of most of the surrounding buildings turned into smithereens. Even some doors came off the hinges," witnesses told Geo TV.
The injured were taken to Abbasi Shaheed Hospital, Patel hospital and Jinnah Hospital.
Aftab Khatri, health secretary of Sindh province, told Xinhua the toll may rise as many of the injured, including a large number of women and children, were in a critical condition.
No group has claimed responsibility for the blasts.
President Asif Ali Zardari and Prime Minister Raja Pervez Ashraf condemned the blasts and directed the hospitals to provide best possible medical treatment to the injured.
A one-day mourning has been announced in the province Monday. The national flag would fly at half-mast on all state-owned buildings across the province.
According to state-run Radio Pakistan, Zardari and Ashraf said such cowardly acts cannot deter Pakistan's determination to eliminate terrorism from the country.
Sindh Chief Minister Syed Qaim Ali Shah announced Rs.1.5 million each for the kin of those who died in the blasts and Rs.1 million for each of the injured.