Several thousand residents of Lianyungang, 480km north of Shanghai, protested outside government offices at the weekend, eyewitnesses told the AFP news agency.
They were worried about the health implications of a nuclear waste reprocessing plant they believe may be built there.
French nuclear group Areva agreed in 2012 to cooperate with state-run China National Nuclear Corp (CNNC) to build a reprocessing facility in China without stating the location.
Local people believe that Lianyungang, a port city in Jiangsu province, is a prime candidate because CNNC is building a large nuclear power station there.
Police deny killing
Pictures posted online show demonstrators on a square surrounded by hundreds of police.
On Monday city police used a social media account to deny a "rumour" that they had beaten protesters, resulting in one of them dying.
China is vastly increasing its nuclear capacity and encouraging state-run firms to build plants abroad.
There are 34 nuclear reactors operating in mainland China, 20 are under construction and work is due to start on more, accordiing to the World Nuclear Association.
But there has been opposition, especially since Japan's Fukushima disaster in 2011.