The Municipal Work Safety Administration said workers at the port failed to fully shut down a valve while loading a dangerous petrochemical, C9, on Thursday.
Xinhua said local residents reported a smell near the port at 6:30 p.m. Thursday. Shortly thereafter, authorities discovered the leak.
Police said they took four people into custody in connection with the incident.
Xinhua said three companies involved in the spill have broken regulations for the sale, transport and loading of the chemical in the past.
Authorities have sent 20 ships to clean up the chemicals and two water plants nearby have shut down, Xinhua reported.