The area resembled a war zone as a video footage posted on social media showed enormous amount of devastation with floors of several buildings blown off due to the explosion.
Ningbo is one of the major international cargo and container shipping ports in the country.
Scores of vehicles parked in the area were damaged.
Rescuers were seen trying to pull people out of the rubble from flats whose roofs were blown off.
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The accident took place at 8:50 AM (local time) in Jiangbei district, state-run Xinhua news agency reported.
Several buildings saw their roofs collapse and windows shatter due to the blast whose cause was not immediately known.
Many residential communities were affected by the blast which shattered windows of buildings up to a kilometre from the blast site. The cause of the explosion was unknown.
Officials said that two persons were killed and more than 30 others taken to hospitals.
He said the blast took place inside a roadside house that was used for commercial purposes and was due to be demolished soon.
A restaurant owner near the explosion site, surnamed Dong, told the Post that his shop was about 500 metres away from the explosion site and all windows facing the scene of the blast had been damaged.
A staff at a hotel at about a kilometre away from the explosion site said that "dozens of windows" were shattered in the explosion.
Two blasts in the port city of Tianjin in 2015 killed 173 people including firefighters and injured hundreds.
A majority of those killed were firefighting crews who went to douse the flames.
The blasts also damaged 304 buildings, 12,428 cars, and 7,533 containers, incurring economic losses amounting to 6.87 billion yuan (USD 1.01 billion).
A year later, courts in Tianjin municipality jailed 49 people, including 25 government officials, for the deadly warehouse blasts.