The bomb -- an improvised explosive device -- was strapped to a motorcycle and detonated remotely by an unknown assailant, Geo TV News reported.
"There were two blasts but it was not clear whether the second was also a bomb," IG of Police Sindh Fayaz Lughari said.
"So powerful was the explosion that it blew off the facades of several flats facing the explosion site. Windowpanes of most of the surrounding buildings turned into smithereens. Even some doors came off the hinges," police added.
Local residents said they began pulling bodies out of the rubble before rescue crews showed up.
"Lawmen including Rangers are nowhere near the site of tragedy. They are shy of facing the bereaved and the catastrophe which befell them. We can see them deployed at a distance," one witness told Geo TV News.
The blasts ripped through a densely populated area of the Abbas Town area where a number of restaurants and tea shops were located, police said.