While one person was killed, around 15 were injured as several of them tried to escape the inferno by jumping off the building.
According to Chief Fire Officer, Ehtishamuddin Siddiqui, around 100 people were still trapped inside the building as fire fighters tried to put out the blaze caused by electrical circuits and wires.
He said the fire department has already declared the fire to be of category three, the highest and most dangerous level.
"Firefighters have got inside the building now and are trying to locate the workers to rescue them," he added.
Police said the fire broke out apparently due to short circuit because of heavy rains and the workers that included panicked women tried to jump off the roof.
"The injured include three women as they also tried to escape by jumping off the roof," one police official said.
Fire officials said 25 people had been rescued so far and the injured have been sent to hospitals for treatment.
The Karachi incident comes on a day when a major fire at a shoe factory in Lahore has claimed as many as 25 lives.
