Civil rights campaigners in the US have begun a 40-day march to highlight what they say is a fresh attack on equal rights for African Americans.

They set out from Selma, Alabama - the starting point 50 years ago for a march in support of watershed legislation enabling black people to vote.

Activists say a 2013 Supreme Court decision has allowed some states to reverse some of that progress.

They hope thousands will join a final rally in Washington DC in September.

America's Journey for Justice will take an 860-mile (1,385 km) route passing through Georgia, South Carolina, North Carolina and Virginia.

Organisers say the outcry triggered by the recent police killings, including the shooting of a black teenager in Ferguson, Missouri, needs to be channelled into a long-term commitment to bring about change, Reuters reports.

"We can continue to be serially outraged, or we can engage in an outrageously patriotic demonstration with a commitment to bringing about reform in this country," said Cornell William Brooks, leader of the National Association for the Advancement of Colored People.

Marchers sang as they crossed the Edmund Pettus bridge, where state troopers beat activists protesting about the death of a black man at the hands of a white police officer in March 1965.

That event, and a follow-up march from Selma to Montgomery led by Martin Luther King helped build momentum for Congress' approval of the Voting Rights Act that removed all barriers preventing African-Americans from registering as voters.

In March, President Barack Obama visited Selma to pay tribute to the original marchers.

He called them "heroes" and said that they had "given courage to millions".

Despite progress, he said, the fight against racism was not over.

"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," the president said.

He also condemned new attempts by state governments to restrict voting rights.