Thirty-one people are in police custody following last week's violent protest against pollution concerns at a solar panel plant in east China's Zhejiang Province, said local authorities Tuesday.

The suspects committed acts of violence and seriously disturbed public order, Shen Xianghong, vice mayor of the city of Haining, said at a press conference held Tuesday afternoon by the city government.

From Thursday to Saturday, more than 500 people from Hongxiao Village in Haining gathered in front of a Zhejiang Jinko Solar Co. factory, demanding an explanation for the mass deaths of fish in a nearby river in late August.

During the protest, eight civilian cars and four police vehicles were damaged, Shen said.

By 8:30 p.m. Monday, the public security bureau of Haining had detained 31 suspects and carried out legal education to another 100 people charged with minor offenses, he said.

Several waste-generating production lines in the factory have halted production as ordered by the city's environmental protection bureau.

The company was also fined 470,000 yuan (73,600 U.S. dollars) for polluting the environment, Shen said.

An initial investigation conducted by the local environmental protection bureau found that the plant failed to properly dispose of solid waste generated during its production of silicon wafers.

The fish were killed by excessive levels of fluoride after waste was swept into the river by heavy rains, according to the investigation.

The bureau ordered the plant to dredge the parts of the river that were contaminated and clean up the polluted water. It also urged the enhancement of waste treatment measures to prevent similar incidents from occurring in the future.

Chen Hongming, deputy head of the environmental protection bureau, said that the factory's waste disposal system has failed regulatory checks since April, adding that the bureau has repeatedly warned the factory about its pollution problem.

Zhejiang Jinko Solar Co., founded in 2006, is a subsidiary of Hong Kong-invested JinkoSolar Holding Co. (NYSE Stock Code: JKS), and has been listed on the New York Stock Exchange since 2010.

The company produces mono- and multi-crystalline photovoltaic panels,cells, and wafers, and sells the solar products globally. It employs more than 10,000 professionals in two factories in east China and has offshore offices and warehouses in the United States and Europe, according to the company's website.