An initial investigation showed the explosion occurred after workers accidentally breached a propylene pipeline beneath the factory in downtown Nanjing, the capital of Jiangsu province in eastern China, state broadcaster CCTV reported.
Large amounts of the flammable toxin apparently leaked into the Nanjing No. 4 Plastics Factory.
CCTV did not give an exact figure on the number of injured. A statement issued earlier by the Nanjing city government said 28 were hospitalized with serious injuries in addition to five people killed outright. Seven of those injured apparently died later of their wounds, although the city government issued no updated figures.
Scores of others received treatment for head injuries, fractures and burns, the official Xinhua News Agency quoted the president of the Nanjing Gulou Hospital, Ding Yitao, as saying.
Xinhua reported buildings and vehicles within 100 yards (meters) of the blast were badly damaged and passengers on a passing bus also injured. Xinhua quoted local residents as saying the explosion felt like an earthquake striking, while those killed included a man living in a nearby home where the ceiling collapsed.
Accidents are common in China's factories, where a lack of attention to safety leads to fires, explosions and other problems killing thousands of people each year.