Edhi ambulances and other rescue workers shifted the dead and the injured to the Jinnah Postgraduate Medical Centre and Civil Hospital. A few of them were also shifted to Abbasi Shaheed Hospital.
Police said that the two bombs, seemed to have been fitted in two motorcycles parked outside the restaurant - Binori Nimko Centre - in the close vicinity of a religious seminary and the Site Police Station, went off one after the other within a short interval at about 7:10 pm.
Eyewitness said that the motorbikes had Kasur and Rawalpindi registration plates.
The bomb disposal squad defused a third bomb moments before it was to explode, Karachi Police Chief Tariq Jameel said.
DIG Operations said that it would be too early to tell anything about the culprits and to assign reasons to this incident. However, he added, he did not think it was an act of sectarian hostility.
Immediately after the blast, terror gripped the entire area and people sitting near the restaurant ran towards the seminary and some of them entered the nearby police station.
A rumour of fourth bomb created the scare among the people and within minutes the entire area was deserted.
On Saturday, there was also an explosion in the city killing two people and wounding three others.
Meanwhile, Sindh Governor Dr Ishratul Ibad has expressed condolence over the loss of lives in the Sunday bomb blasts.
He prayed for the departed souls and grant of courage and strength to bear the loss with fortitude while wishing early recovery of the injured.
The Governor, on behalf of the Sindh government, also announced compensation for the victims of the blast.
Chief Minister Dr Arbab Ghulam Rahim has also condemned the incident and has termed it a cowardly act of terrorism. He has expressed his sorrow and grief and sympathised with the families of those killed and injured.
He has directed the concerned authorities to provide appropriate medical facilities to the injured at government's expense.
The chief minister has asked Sindh Police to intensify its patrolling, checking of suspects and to take help of the community in the discharge of its duty.
Agencies add: Doctors and police said eight people were killed and more than 50 injured were taken to hospitals.
Police and paramilitary rangers threw a cordon around the site of explosion.
"There was an explosion in a restaurant close to Jamia Binoria seminary which was followed by another big blast caused by a bomb fitted in a motorbike parked outside the restaurant," city police chief Fayyaz Leghari told AFP.
An AFP correspondent saw the bodies of four seminary students in Abbasi Shaheed Hospital.
Dr Syed Mazhar said two of the injured died in the hospital. A six-year-old child is also among the dead. Some of the injured are in critical condition, he added.
Hayan Ahmed, an 18-year-old seminary student, said he was taking snacks and tea at the restaurant when the first bomb went off and as he came out he was injured by the second blast.
Mohammad Yar, a teacher at the seminary, told reporters outside the hospital: "Madrassa was the target, but at this stage we don't want to blame anyone."
Yar said it is the responsibility of the government to arrest the culprits.
"Every second day there have been blasts in Shia and Sunni mosques, Islamic seminaries and other places, so to whom we should blame?" he said.