William M. Welch, and Yamiche Alcindor

USAToday

A federal judge ruled Monday that a tactic police used to control protesters in Ferguson, Mo., is unconstitutional and issued a preliminary injunction halting the practice.

U.S. District Judge Catherine Perry in St. Louis ordered law enforcement agencies to stop enforcing a requirement that protesters keep moving rather than stand still.

Law enforcement officers, seeking to control angry crowds, had ordered people to walk and not stand still while demonstrating over the shooting death of Michael Brown by a Ferguson police officer. The American Civil Liberties Union sued to halt the practice.

Perry wrote in her order that the requirement violates the constitutional rights of protesters. She said she granted a preliminary injunction "because it is likely that these agencies will again apply this unconstitutional policy.''

The judge cautioned, however, that police still can enforce Missouri's failure-to-disperse law and other laws to control crowds and protect people and property.

"This injunction prevents only the enforcement of an ad hoc rule developed for the Ferguson protests that directed police officers, if they felt like it, to order peaceful, law-abiding protesters to keep moving rather than standing still,'' she wrote.

The ACLU and Amnesty International USA hailed the ruling as a victory.

"Vague rules that are applied in a haphazard fashion tend to increase community tension," said Tony Rothert, legal director of the ACLU of Missouri. "Judge Perry's injunction is a huge win for peaceful protesters and those who believe in the rule of law.''

The Missouri Highway Patrol, which was placed in charge of policing protests in Ferguson, said in a statement that its goal "has been to allow citizens to speak while keeping the community safe.''

"Today's ruling is consistent with these principles because it allows protesters to exercise their constitutional rights to peaceably assemble but also allows law enforcement to impose appropriate restrictions to protect the public from violence,'' the Highway Patrol statement said.

Perry's order stops police "from enforcing or threatening to enforce any rule, policy or practice that grants law enforcement officers the authority or discretion to arrest, threaten to arrest or order to move individuals who are violating no statute or regulation and who are peaceably standing, marching or assembling on public sidewalks in Ferguson, Missouri.''