US President Barack Obama has marked the 50th anniversary of the Selma civil rights march in Alabama by paying tribute to the "heroes" who took part.
He delivered a speech commemorating "Bloody Sunday" on 7 March 1965, when security forces attacked black demonstrators in the city.
Mr Obama said the marchers, who were campaigning for equal voting rights, had "given courage to millions".
His wife Michelle and about 100 members of Congress also attended the event.
"Because of what they did, the doors of opportunity swung open not just for African-Americans, but for every American," he said, standing in front of the Edmund Pettus Bridge where the violence took place.
Police beat and used tear gas on demonstrators as they made their way over the crossing, on the day that became known as "Bloody Sunday".
That event, and a follow-up march from Selma to Montgomery two weeks later, helped build momentum for approval of the Voting Rights Act by Congress later that year.
The legislation, pushed by President Lyndon Johnson, removed all barriers preventing African-Americans from registering as voters.
'Sweat and tears'
Mr Obama reminded the American public that despite progress the fight against racism was not over.
He addressed the recent police killings of unarmed black men and teenagers, which had triggered protests in several US cities.
"This nation's long racial history still casts its long shadow upon us. We know the march is not yet over, the race is not yet won," he said.
His comments followed demonstrations in the northern state of Wisconsin on Friday, triggered by reports that a black man had been shot by a police officer.
The 19-year-old, identified as Tony Robinson, was unarmed according to police. It is alleged he assaulted the officer before shots were fired.
Mr Obama also condemned new attempts by state governments to restrict voting rights.
"Right now, in 2015, 50 years after Selma, there are laws across this country designed to make it harder for people to vote. As we speak, more of such laws are being proposed," he said.
"Meanwhile, the Voting Rights Act, the culmination of so much blood and sweat and tears... stands weakened, its future subject to partisan rancour."
Georgia congressman John Lewis, a Democrat who was among those injured in the violence 50 years ago, also addressed the crowd of more than 40,000.
He said: "I want to thank each and every one of you who marched across the bridge on Bloody Sunday. You didn't have to do but you did.
"Six hundred people marched into history. We were so peaceful, so quiet. No-one saying a word.
"We were beaten, tear gassed, some of us were left bloody here on this bridge. But we never became bitter or hostile."
President George W Bush - who was in office between 2001 and 2009 - also took part in the commemorations.