Tens of thousands of Americans of all backgrounds and colors thronged to the National Mall to join the nation’s first black president and civil rights pioneers in marking the 50th anniversary of Martin Luther King Jr.’s “I Have a Dream” speech. President Barack Obama urged the crowd to become modern-day protesters for economic justice and racial harmony. “The arc of the moral universe may bend toward justice but it doesn’t bend on its own,” Obama said, in an allusion to King’s own message.