HANGZHOU, Dec. 11 -- Nearly two dozen people were caught by police for polluting a river with toxic sludge in east China's Zhejiang Province, authorities said Thursday.

According to police of Haining City, the group of 23 was operating a company that illegally treated sludge from local leather printing and dyeing firms in Haining and the neighboring Tongxiang City since May 2012.

The company, which rented docks on the river in Tongxiang, poured about 300,000 tonnes of sludge containing a large amounts of heavy metals into the Grand Canal, a waterway running from Beijing to Hangzhou in Zhejiang, and Poyang lake in Jiangxi Province.

Police in Haining started the investigation in June when the water quality in some local rivers was found deteriorating. They suspected the pollution was caused in the upper reaches in Tongxiang.

The suspects were caught in the provinces of Jiangxi, Zhejiang and Jiangsu and the city of Shanghai. Among them, 18 have been detained.

In addition, six boats used to transport the sludge were seized by police. The case is still under further investigation.