ST. LOUIS • Police may not force peaceful, law-abiding protesters to keep moving, a federal judge said Monday, because it violates their constitutional rights.

U.S. District Judge Catherine Perry issued a preliminary injunction ordering police to stop using a crowd-control tactic that was intended to enforce curfew during the most volatile nights of the Ferguson protests.

The ACLU last month asked the judge to put a stop to the practice of forcing protesters to “keep moving” and not stand on sidewalks. Police began using the rule Aug. 18 along West Florissant Avenue in Ferguson, during protests and riots that followed the shooting of an unarmed black teenager by a police officer there.

The injunction “is a huge win for peaceful protesters and those who believe in the rule of law,” Tony Rothert, legal director of the ACLU, said in a news release.

Perry’s injunction said the practice police adopted, which some activists called the “five-second rule,” violated free speech rights. The judge also said the policy violated due process rights because “its enforcement was entirely arbitrary and left to the unfettered discretion of the officers on the street,” effectively making them lawmakers and enforcers.