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.