Thousands of people have crowded on and around an Alabama bridge, commemorating bloody clashes 50 years ago between police and demonstrators during the civil rights struggle.

Associated Press reporters at the scene in Selma said thousands jammed shoulder to shoulder, many not moving as they held up signs and some chanted or sang hymns. They said some groups managed to cross earlier.

"Black lives matter, all lives matter," read one sign raised in the crowd, which was commemorating the anniversary.

Police estimated at least 15,000 to 20,000 people were present.

On March 7, 1965, police beat and tear-gassed marchers at the foot of the bridge in Selma in a spasm of violence that shocked the nation. The attack help build momentum for passage of the Voting Rights Act later that year.