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Tens of thousands of people are descending on Washington's National Mall today to commemorate the 50th anniversary of the Rev. Martin Luther King Jr.'s "I Have a Dream" speech and the March on Washington that took place half a century ago. President Obama will address the crowd this afternoon from the Lincoln Memorial.

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The event will also feature members of the King family, lawmakers and civil rights leaders, including Rep. John Lewis, D-Ga., who spoke at the 1963 march and will do so again today. There will be more than a few celebrity appearances, too.

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The event is being organized by The King Center and The Coalition for Jobs, Justice and Freedom.

INTERACTIVE: Prominent Americans' View on the 'Dream'

Get the latest at our liveblog of the 50th anniversary of the March on Washington. Updates will appear below automatically when the event starts or at this link if you are on a mobile device.