SELMA, Ala. » President Barack Obama, first lady Michelle Obama and about 100 members of Congress are converging on Selma, Alabama, on Saturday for the 50th anniversary of a landmark event of the civil rights movement.

Obama will speak in the riverside town to commemorate "Bloody Sunday," the day in 1965 when police attacked marchers demonstrating for voting rights.

The violence preceded the Selma-to-Montgomery march, which occurred two weeks later. Both helped build momentum for congressional approval of the Voting Rights Act later that year.

Thousands of people attend the annual observance of the anniversary, and organizers expect an even larger crowd this year.

First lady Michelle Obama will travel with the president, and former President George W. Bush also plans to attend. The congressional delegation will include U.S. Rep. John Lewis of Georgia, an Alabama native who was among the marchers seriously injured in the violence 50 years ago.

More events are planned for Sunday, with civil rights veterans leading a symbolic walk across the Edmund Pettus Bridge. Police beat and tear-gassed marchers at the foot of the bridge on March 7, 1965 in an ugly spasm of violence that shocked the nation.

Today, Selma still struggles to overcome its legacy.

The city’s population has declined by about 40 percent to 20,000 in the last 50 years and Dallas County’s unemployment rate is nearly double the state average. Public schools in Selma are nearly all black; most whites go to private schools. Blacks lead the annual "Bloody Sunday" commemoration; whites lead an annual re-enactment of the 1865 "Battle of Selma" to attract Civil War re-enactors.

For Obama, the trip to Selma marks the continued celebration by the first black U.S. president of three of the most important civil rights milestones in America’s tortured racial history.

In 2013, Obama spoke at the 50th anniversary celebration of Martin Luther King’s "I Have a Dream" speech on the steps of the Lincoln Memorial. Last year, he addressed the 50th anniversary of the signing of the 1964 Civil Rights Act by President Lyndon B. Johnson.

On Saturday, Obama will lead a tribute at the Edmund Pettus Bridge to mark the 50th anniversary of what became known as "Bloody Sunday," when police set upon scores of people marching from Selma, Alabama, to Montgomery, Alabama, to protest their inability to vote, clobbering and tear-gassing them until they were bloody.

The Obamas will be accompanied by their daughters Malia and Sasha. After the remarks, Obama and the first lady will join marchers in a recreation of the bridge walk.

Obama said last week that the family was coming to pay tribute "as Americans to those who changed the course of history" at the bridge.

"Not just the legends and the giants of the Civil Rights Movement like Dr. King and John Lewis, but the countless American heroes whose names aren’t in the history books, that aren’t etched on marble somewhere — ordinary men and women from all corners of this nation, all walks of life, black and white, rich and poor, students, scholars, maids, ministers — all who marched and who sang and organized to change this country for the better," Obama said at a Black History Month observance at the White House.

Obama’s Selma remarks are expected to touch on the issue of voting rights. Obama also addressed the issue in his State of the Union address. His administration has challenged Southern states that have imposed new voting requirements, including showing picture identification before being allowed to vote and curtailing opportunities to vote early. Critics of these moves say they disenfranchise mostly minority voters and set back the gains won by civil rights marchers, including those who crossed the Edmund Pettus Bridge.

A divided U.S. Supreme Court voted 5-4 in June 2013 to remove from federal law the most effective tool for fighting discrimination against voters. Ruling in a case from Shelby County, Alabama, the high court eliminated the Justice Department’s ability under the Voting Rights Act to identify and stop potentially discriminatory voting laws before they take effect.

Associated Press reporter Darlene Superville contributed from Washington.