In the U.S. state of Missouri, hundreds of protesters have taken to the streets of city of Saint Louis after a judge ruled that a white former police officer was not guilty of murder in the 2011 shooting of a black man.

Protesters marched peacefully in downtown chanting “No justice, no peace” and “Shut it down.” Authorities put the National Guard on standby and appealed for calm. Local schools planned early dismissals and businesses shut down.

Jason Stockley was acquitted of first-degree murder for killing the 24-year-old Anthony Lamar Smith. The former policeman was accused of planting a gun in Smith’s car but testified he acted in self-defense.

The case has drawn intense attention in St. Louis, where racial tensions have been high since the 2014 killing of a black man in the suburb of Ferguson by a white police officer.