Chinese authorities are vowing to inflict what they call “severe punishment” on organizers of a two-day protest who are concerned about the environmental impact of a proposed heavy metal refinery in their town.

Authorities say unrest broke out Monday when several thousand protesters clashed with police outside a government building in the small central city of Shifang. Police responded with tear gas, and pictures circulated online showed police beating protesters. Officials say 13 people were injured.

Shifang police on Tuesday demanded the surrender of those organizing the demonstrations, saying anyone using the Internet or text messages to organize the “illegal protests” should immediately stop their activities.

The Shifang government said Monday it has suspended the construction of the molybdenum and copper refinery, which citizens say would cause heavy amounts of pollution.

Such localized protests are increasingly common in China, where city officials have struggled to balance rapid urban development with rising public anger over environmental threats.