The explosion happened at around 9 a.m.(01:00 GMT) at a factory in Jiangbei district in Ningbo, a coastal city about 100 kilometres (62 miles) south of Shanghai, according to state news agency Xinhua, citing local authorities, witnesses and the Ningbo city’s fire department.
In a separate report, China Central Television (CCTV) did not refer to a factory, but said the force of the explosion shattered windows and punched holes in the walls of buildings as much as a kilometre away.
CCTV images showed a few flattened cars and a low-rise building with a collapsed wall. The state broadcaster said it was not a gas explosion, without elaborating further.
CCTV said two people were killed while two others who had been injured were in serious condition.
According to Zhejiang Daily, several buildings collapsed in a nearby residential area that had already been cleared of people. Police told Reuters the area had been marked for demolition.
Rescue work and investigation into the cause were underway, police said in a statement issued on its verified Weibo account.
Blasts and other accidents are common in China due to patchy enforcement of safety rules, although the government has pledged to improve checks to stamp out such incidents.