DeRay Mckesson, one of the best known voices for the Black Lives Matter movement, was among hundreds of people arrested at weekend demonstrations across the country. He spent 16 hours in a Baton Rouge jail cell until he was released Sunday afternoon, vowing to continue demonstrating because he was convinced that the authorities want activists to be “too afraid to protest.”

Mr. Mckesson, in a telephone interview shortly after his release, said he believed that his arrest was unlawful and that the police unfairly conducted mass arrests while people were peacefully assembled and out of the way of traffic along a highway.