BIRMINGHAM, Ala. (Reuters) - Fifty years after a bomb ripped through a church basement, killing four girls and shocking a racially divided nation, the city of Birmingham, Alabama, on Sunday commemorated the tragedy that marked a shift in the country's battle over civil rights.

"It is a sad story, but there is a joy that came out of it," said Sarah Collins Rudolph, who survived the blast at the 16th Street Baptist Church. Her 14-year-old sister, Addie Mae Collins, was among the victims of the bomb planted by a member of the Ku Klux Klan.

The young girls' deaths mustered support for passage of the Civil Rights Act of 1964, which outlawed discrimination on the basis of race, ethnicity, religion or gender. The act also ended the Jim Crow laws that had enforced rigid segregation practices across much of the southeastern United States.

Rudolph lost an eye and was partially blinded in her remaining eye when the bomb went off, while she and four other girls were in a church restroom.

"I will never forget walking over their dead bodies," she told Reuters.

At 10:22 a.m. Central Time (1522 GMT), the time of the blast, the church's bell tolled in remembrance of Collins, 11-year-old Denise McNair, and Carole Robertson and Cynthia Wesley, both 14.

The bell-ringing was part of a day of activities throughout the city remembering the tragedy and celebrating the Civil Rights Act.

An afternoon service at the church ended with people holding hands and singing "We Shall Overcome," a civil rights anthem.

Speaking at the service, U.S. Attorney General Eric Holder said: "Millions called for - and helped to secure - the landmark Voting Rights Act of 1965, which empowered the Justice Department to fight unjust attempts to abridge voting rights and restrict access to the franchise. This is a fight we will continue."

President Barack Obama's administration has vowed to challenge existing voting laws it says discriminate by race, an effort to counter a U.S. Supreme Court ruling in June that freed states from the strictest federal oversight under the Voting Rights Act.

Shelby County, part of the Birmingham metropolitan area, filed the lawsuit that challenged the Voting Rights Act. As a result of the Supreme Court decision, Alabama will institute a voter identification law in 2014 that critics say will suppress the minority vote.

Holder has said the Justice Department will ask a federal court for renewed power to block new election laws it says illegally discriminate against blacks and other minorities.

RICE LOST HER CHILDHOOD PLAYMATE

Among those attending events in Birmingham were former U.S. Secretary of State Condoleezza Rice, who was a childhood playmate of one of the victims, former Atlanta Mayor Andrew Young, filmmaker Spike Lee and Bernice King, the daughter of Martin Luther King.

Rice, who was a playmate of McNair's, recalled the fear that followed the blasts, which she said influenced her work.

"I would often encounter people dealing with long, long grievances," Rice said on Sunday. "I would tell them my parents couldn't even take me to a movie theater. But even though I couldn't eat at the lunch counter at Walgreens, they convinced me I really could be the president."

The last surviving bomber, Thomas Blanton, 83, sits alone in a prison cell not far from Birmingham. Since his 2001 conviction, his list of visitors has dwindled to his daughter and a few others, said Brian Corbett, spokesman for the Alabama Department of Corrections.

Blanton shows no remorse, said Doug Jones, the U.S. attorney who prosecuted Blanton and Bobby Frank Cherry years after the bombings.

Celebrated as martyrs in the history of civil rights, the four bombing victims were honored last week with a Congressional Gold Medal, the highest honor bestowed on civilians.

On Thursday, the families were given replicas of the medal, which pictures the four girls, the church and their names. In the center the medal reads: "Pivotal in the struggle for equality," said U.S. Treasurer Rosie Rios.

Back in Birmingham on Sunday, Pastor Arthur Price asked a Sunday school class, "What would you do if you could get your hands on that Blanton dude who bombed the church?"

The Christian answer, he said, is to practice "the love that forgives." The same Bible lesson was being taught 50 years ago, when the bomb exploded.

(Editing by Daniel Trotta and Stacey Joyce)