Initial reports said 68 people were injured but Turkish television put the number at 117.
City Gov. Muammer Guler said there were no immediate indications the blast was linked to a terror attack, the Hurriyet news agency said.
"It's certain that the explosion was related to a sparkler, torch or fireworks," Guler said.
Four of the injured were in critical condition, he said.
Witnesses told CNN Turk television the explosion happened on the third floor around 9:30 a.m. Thursday. That blast apparently triggered a fire and second explosion.
The building in the Zeytinburnu industrial suburb also houses textile and paper processing companies, the New Anatolian said.
Putin thinks Obama would save him if he were drowning
Chipotle plans first price increase in 3 years