Pakistan's economic nerve centre of Karachi has come to a standstill following the overnight car bomb blast which killed at least 45 people.

There are few vehicles on the roads as schools, colleges and markets remain closed to mourn the dead.

The explosion ripped through the densely populated Abbas Town, a Shiite-dominated area in the city, leaving a wide area of devastation. The blast took place between two apartment blocks located at the entrance to Abbas Town.

According to Dawn news site, 40 to 50 apartments and at least 10 shops have been destroyed in the blast which has left a huge crater.

As many as 150 people injured in the blast are undergoing treatment in hospitals.

Many people are still feared trapped under the rubble as rescue workers are finding it difficult to remove heavy concrete.

No militant group has so far claimed responsibility for the bombing. The authorities are reported to be probing all angles, including whether it was a suicide attack.

"Over 150kg of high explosives laced with ball bearings were detonated by a time device," senior police official Aleem Jafri told Dawn.

Some reports suggest that there was a second blast, but this was not officially confirmed by the authorities.

"It's like doomsday to me. I was watching television when I heard an explosion and my flat was badly shaken. I saw people burning to death and crying with pain. I saw children lying in pools of their own blood and women running around shouting for their children and loved ones," eyewitness Mariam Bibi told Reuters.

The attack has been condemned by top political and religious leaders.

The Shiite community has been the target of frequent attacks by Sunni extremists in Pakistan in recent days killing dozens of people. The Sunni militant group al-Qaida-linked Lashkar-e-Jhangvi had claimed responsibility for earlier attacks.

In early January, more than 200 people were killed in a targeted attack on Shiite Muslims in Quetta region.