The US Supreme Court on Monday allowed a Black Lives Matter activist to be sued by a Louisiana police officer injured during a protest in 2016 in a case that could make it riskier to engage in public demonstrations, a hallmark of American democracy.

In declining to hear DeRay Mckesson’s appeal, the justices left in place a lower court’s decision reviving a lawsuit by the Baton Rouge police officer, John Ford, who accused him of negligence after being struck by a rock during a protest sparked by the fatal police shooting of a Black man, Alton Sterling.

The New Orleans-based 5th US Circuit Court of Appeals in 2023 rejected, Mckesson’s defence that his rights to free speech and assembly under the US Constitution’s First Amendment protect him from the negligence claim. Mckesson was represented by lawyers including from the American Civil Liberties Union.

The Baton Rouge protest was one of numerous demonstrations in the United States in 2015 and 2016 arising from incidents involving police and Black individuals. These predated the massive racial justice protests that flared in various cities in the United States and abroad following the 2020 murder of George Floyd, a Black man, by a white police officer in Minneapolis.

The 5th Circuit’s decision to allow Ford’s lawsuit could make it easier to sue protest leaders for the illegal conduct of an attendee – an outcome that, according to some legal scholars, could stifle activism seeking political or societal change.

Sterling was shot by a Baton Rouge police officer on July 5, 2016, after a struggle outside a convenience store where he was selling homemade CDs. The death inflamed racial tensions in the city. The protest four days later demanding accountability took place in the area in front of police headquarters.

Ford was among the officers assigned to make arrests of protesters on a public highway. He was struck in the face by a rock or piece of concrete hurled by an unidentified person, losing teeth and suffering head and brain injuries, according to his lawsuit.

Ford’s lawsuit, seeking monetary damages, argued that McKesson should have known from his actions leading the protest that it would turn violent. Mckesson was arrested on the day of the protest but the charge was later dropped.

US District Judge Brian Jackson dismissed Ford’s suit in 2017. But the 5th Circuit in 2023 revived it, finding that the First Amendment did not bar the negligence claim.

The 5th Circuit rejected an argument that the lawsuit was foreclosed by a 1982 Supreme Court ruling involving Black civil rights activists in Mississippi limiting liability for protest leaders over the conduct of others when they are involved in actions protected by the First Amendment.

A dissenting 5th Circuit judge, Don Willett, invoked Dr Martin Luther King Jr’s march in 1965 from Selma to Montgomery, Alabama, drawing attention to the disenfranchisement of black voters. Political uprisings, from picketing to riots, have “marked our history from the beginning,” Willett noted.

The 5th Circuit majority’s “theory would have enfeebled America’s street-blocking civil rights movement, imposing ruinous financial liability against citizens for exercising core First Amendment freedoms,” Willett wrote.

Mckesson’s case had made it to the Supreme Court once before. The justices in 2020 ordered additional review by lower courts to determine whether Louisiana state law allowed for Ford’s claim.

The Black Lives Matter movement was formed after the killing of a Black 17-year-old named Trayvon Martin by a man named George Zimmerman in Florida in 2012. Its activists have criticised overly aggressive policing particularly against Black people.

• A Reuters report