Protesters clashed with police in Atlanta at the site of a training centre which opponents say will fuel police militarisation and also uses valuable forest land.
Hundreds of people attended a concert at the site and police said a group then threw rocks and firebombs at officers.
Thirty-five people were arrested, police said.
Images of the protests on social media showed several fires.
The planned Atlanta Public Safety Training Center, dubbed « Cop City », is being built on land owned by the city in South River Forest.
After the clashes began, police locked down the area and a Swat team was in attendance.
Police said the group of protesters had used the peaceful demonstration as cover to « conduct a coordinated attack on construction equipment and police officers ».
Several pieces of construction equipment were destroyed, the statement said.
« The illegal actions of the agitators could have resulted in bodily harm. Officers exercised restraint and used non-lethal enforcement to conduct arrests, » it added.
