A major explosion hit China's megaport city of Ningbo on Sunday, killing at least two people, injuring dozens and wrecking buildings, state media and local authorities said.

The Ningbo government said on a social media account that the blast occurred in a “vacant lot” but it did not provide details about what may have caused it.

Local authorities said two people died, two were seriously injured and an unidentified number were being treated for injuries in the major port city.

State media said more than 30 people were hospitalised, with the Xinhua news agency reporting that the explosion had taken place in a demolished factory at around 9:00 am.

Industrial accidents are common in China, where safety standards are often lax.

Television images showed cars twisted and mangled by the force of the explosion, a plume of grey smoke rising in the sky, and debris scattered for dozens of metres around the site of the incident.

Aerial images posted by the People's Daily on Twitter showed at least four wrecked buildings around a wasteland of concrete debris.

Footage showed rescuers wearing helmets carrying injured people away from the area, while others stood over a person lying on the ground.

Pieces of concrete, wood and glass were strewn across a large area. Metal gates were twisted open and windows blown out of buildings.

According to CCTV, eyewitnesses said there were “a large number of injured people” in the city, one of China's largest ports, which sits just south of Shanghai.

The Communist Party's People's Daily said on Twitter that no one lived at site of the explosion but garbage collectors might have been working there.

An enquiry has been launched to determine the cause of the explosion, the local police in Jiangbei district, where the blast happened, said on social media.

Ningbo is one of the country's major international container shipping ports.

Past disasters

China has been rocked by several industrial accidents in recent years.

In 2015, giant blasts killed at least 165 people in the northern port city of Tianjin, causing over $1 billion in damage and sparking widespread anger over a perceived lack of transparency by officials about the accident's causes and its environmental impact.

A government enquiry into the Tianjin accident released in February 2016 recommended 123 people be punished.

The official who was mayor at the time of the accident was sentenced to 12 years in prison for graft in September.

Huang Xingguo, 62, had also headed the disaster response committee.