By Jeremy Reynalds
Senior Correspondent for ASSIST News Service
|NY Times Photo|
According to a story by the BBC, the blast in the mainly Shia Muslim area of Abbas Town destroyed several buildings and set others on fire. Some reports spoke of a second explosion.
There's no word yet on who planted the bomb, which went off near a mosque as worshipers left evening prayers.
The BBC said Pakistan's Shia minority are the target of frequent sectarian attacks from Sunni militant groups.
The explosion sent a huge column of smoke into the sky above Karachi and caused a power cut in part of the city.
Police are investigating whether it was a suicide attack.
The BBC said rescuers have been struggling to reach people trapped under the rubble. Residents have been using car headlights to help the search for survivors, local media reports said.
Around 150 people were wounded by the explosion, officials said.
"I was watching television when I heard an explosion and my flat (apartment) was badly shaken," the BBC said Karachi resident Mariam Bibi told Reuters news agency.
"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," she added.
Rescue work was delayed as some residents fired guns into the air in anger at the carnage, reports say.
The BBC reported that Pakistan's main political and religious leaders rushed to condemn the attack - the latest to target the Shia minority.
Nearly 200 people were killed in two separate bombings targeting the Shia community in the south-western city of Quetta in January and February.
The BBC said some relatives of the victims there initially refused to bury their dead in protest at what they said was the failure of the authorities to protect their community from attack.
According to the BBC, no group has yet admitted to carrying out the Karachi bombing, but correspondents say suspicion is likely to fall on Sunni militant groups.
Groups such as Lashkar-e-Jhangvi regard Shia Muslims as heretics and have stepped up attacks in recent years.
The BBC reported they are thought to have set several training camps for militants and police seizures have shown they have access to large quantities of weapons and explosives, the BBC's M Ilyas Khan in Islamabad said.
The BBC said some activists called 2012 the worst year in living memory for attacks on Pakistan's Shia community.
But already this year bombings in the south-western of Quetta alone have killed nearly 200 people.
Last month, the BBC reported, Pakistan's Supreme Court called on the authorities to devise a strategy to protect Shia Muslims more effectively, given the increase in attacks.
The BBC said Karachi - Pakistan's biggest city and commercial capital - has a long history of violence.
As well as a sectarian divide between Sunni and Shia, that city has also seen conflict between different ethnic communities - Pashtuns from north-west Pakistan, Mohajirs (immigrants from India following the Partition) and Sindhis.