At the dedication of the memorial to Dr. Martin Luther King Jr. in Washington on Sunday, President Obama said the slain civil rights leader made the Union "more perfect."

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Demonstrators rally under the new Martin Luther King Jr. memorial in Washington, D.C., on Saturday.

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Andrew Young (top right) speaks about the significance of the Rev. Martin Luther King Jr. at the groundbreaking for the memorial in Washington, D.C., on Nov. 13, 2006. Also present are other King lieutenants, including the Rev. Jesse Jackson (top center) and Georgia Rep. John Lewis (leaning on shovel). Lauren Victoria Burke/AP hide caption

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The Martin Luther King Jr. Memorial opened to the public on Monday. It will be officially dedicated on Sunday. Allison Keyes/NPR hide caption

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The memorial to the Rev. Martin Luther King opens on the National Mall this week after 25 years of planning and more than $100 million. Allison Keyes/NPR hide caption

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James Earl Ray was sentenced to 99 years in prison for the murder of Martin Luther King Jr. He died in prison in 1998. Hulton Archives/Getty Images hide caption

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The poster used to advertise King's appearance at Bethel College on Jan. 21, 1960. Courtesy Mennonite Library and Archives/Bethel College hide caption

itoggle caption Courtesy Mennonite Library and Archives/Bethel College