The plant in Zhejiang province produces mainly industrial hydrogen peroxide, which contains arsenic, heavy metals and other toxic ingredients.

Officials were trying to prevent the pollutants from settling in the nearby Ou River, the second largest in Zhejiang, which also passes alongside the cities of Wenzhou and Lishui that have a combined population of 9.5 million.

"We will take all possible measures to minimize the harm," official Luo Yongwen, who was in charge of the emergency, told CCTV.

Smaller explosions continued throughout the day, releasing thick toxic smoke. The fire was only extinguished around 8pm (1200 GMT), about 12 hours after the first blast, China Central Television (CCTV) reported.

Luo said emergency workers would try to direct liquid pollutants into a waste water treatment plant in the factory. They would also use in the water active carbon, an absorbant that can attract thousands of different chemicals.

He said the water quality in the Ou River within five kilometers (three meters) from the accident site was Thursday up to national safety standards.

Industrial pollution of China's waterways was thrown into the global spotlight last November when a chemical factory explosion in the northeastern province of Jilin released 100 tons of benzene and nitrobenzene into the Songhua river.

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