An explosion in a port city south of Shanghai on Sunday killed two people and injured at least 30 others, knocked down buildings and left streets littered with damaged cars and debris, the government and news reports said.

The early morning explosion struck a riverfront neighborhood in Ningbo, one of China's busiest ports, the official Xinhua News Agency and other outlets reported.

The cause of the blast at a factory premises was not immediately clear.

Initial media reports said there was an explosion in a building in the district of Jiangbei which was due to be razed to the ground.

Several buildings in the area collapsed and window panes were shattered within a one-kilometer radius as a result of the blast, the South China Morning Post reported.

"The explosion was like an earthquake," said the owner of a nearby restaurant, who added that the shock was so great that one of his clocks stopped at exactly 8:55 am (0055 GMT).

Onlookers also reported the sky being filled with black smoke after the blast. As many as 16 fire engines were called to the scene, according to media reports.

China has seen a number of industrial accidents, often involving dangerous and explosive materials for construction and demolition being stored improperly.

Two years ago, 173 people were killed in a massive explosion at a warehouse storing dangerous materials in the northern city of Tianjin. Many of those killed were firefighters who had responded to an initial blaze.