Thousands of marchers have kicked off the 50th anniversary events for the March on Washington, honouring the civil rights progress made since the watershed 1963 event.

Many also bemoaned what they see as an attack on that progress since Martin Luther King Jr. delivered his stirring "I have a Dream" speech.

Marchers began arriving early Saturday to gather on the National Mall, many staking out their spots as the sun rose in a clear sky over the Capitol.

Organisers have planned for about 100,000 people to participate in the event, which is the precursor to the actual anniversary of the 28 August, 1963, march.

The March on Washington drew some 250,000 people to the National Mall and ushered in the idea of massive, nonviolent demonstrations and helped bring about the 1964 Civil Rights Act.

Organisers of today's march said hoped this year's event would serve to inspire people again to educate themselves about issues they see as making up the modern civil rights struggle.

They cite a recent Supreme Court ruling that effectively erased a key anti-discrimination provision of the 1965 Voting Rights Act and persistent unemployment among African-Americans, which is about double that of white Americans.

Also looming over the anniversary is the Florida shooting death of unarmed black teenage Trayvon Martin and the acquittal of George Zimmerman.

Today's event is being led by the Reverend Al Sharpton and King's son Martin Luther King III. After several speeches, participants will walk the half-mile from the Lincoln Memorial to the two-year-old memorial.

On the day of the anniversary, US President Barack Obama will speak from the steps of the Lincoln Memorial, the same place King stood when he delivered his "I Have a Dream" speech.

Mr Obama will be joined by former Presidents Bill Clinton and Jimmy Carter.

Churches and groups have been asked to ring bells at 3 p.m. Wednesday, marking the exact time King spoke.