The nation's first African-American president, speaking by the river bridge where Alabama state troopers beat black voting rights marchers 50 years ago today, called for Congress to restore part of the federal law those marchers inspired.

President Barack Obama told a cheering throng that jammed the streets of downtown Selma that the Voting Rights Act "stands weakened" by a 2013 U.S. Supreme Court decision.

That decision, in a case originating in Shelby County, led the court to toss out the part of the law that required Alabama and some other states to get federal clearance before making changes affecting elections.

Obama and first lady Michelle Obama were in Selma for the 50th anniversary of Bloody Sunday.

Former President George W. Bush and Laura Bush shared the stage with the president.

U.S. Rep. John Lewis, a Georgia congressman who was one of the marchers beaten in 1965, introduced the president.

U.S. Rep. Terri Sewell of Birmingham, a Selma native, introduced Lewis.

Gov. Robert Bentley spoke.

The event commemorated "Bloody Sunday," a landmark in the civil rights movement that led to passage of the Voting Rights Act, a law that Obama said has lost its teeth.

"In 2015, fifty years after Selma, there are laws across this country designed to make it harder for people to vote," the president said. "As we speak, more of such laws are being proposed.

"Meanwhile, the Voting Rights Act, the culmination of so much blood and sweat and tears, the product of so much sacrifice in the face of wanton violence, stands weakened, its future subject to partisan rancor."

Noting that about 100 members of Congress were expected in Selma today, the Obama said they should "pledge to make it their mission to restore the law this year."

In 2013, the Supreme Court ruled 5-4 in Shelby v. Holder that the Voting Rights Act's formula used to determine what states had to get pre-clearance for election law changes was unconstitutional.

Shelby County filed the lawsuit against the Voting Rights Act in 2010, claiming that it was unconstitutional to require some jurisdictions to get pre-clearance without evidence that they are still engaged in discriminatory practices.

Chief Justice John Roberts said Congress could fix the law by writing a new formula.

Charles Steele, president of the Southern Christian Leadership Conference, one of the groups that led the Bloody Sunday march, said the president doesn't talk about the Voting Rights Act enough and was glad to hear it highlighted in his speech.

"We've got a watered down version, and we must restore the Voting Rights Act," Steele said. "That's what he said. And I was so impressed with that."

Obama said Lewis, who introduced him, "is one of my heroes."

Lewis was one of the marchers injured in 1965.

"The Americans who crossed this bridge, they were not physically imposing," Obama said. "But they gave courage to millions. They held no elected office. But they led a nation. They marched as Americans who had endured hundreds of years of brutal violence, countless daily indignities -- but they didn't seek special treatment, just the equal treatment promised to them almost a century before."

Obama spoke out this week about a Justice Department investigation into the Ferguson, Mo., police department, saying that racial discrimination there was "oppressive and abusive."

He used today's event in Selma to make a point that the racial climate is much better than in 1965.

"What happened in Ferguson may not be unique, but it's no longer endemic," the president said. "It's no longer sanctioned by law or by custom. And before the Civil rights movement, it most surely was."

But he said it's a more common mistake to think that incidents of racial bias are isolated.

"We know the march is not yet over. We know the race is not yet won," he said.

On March 7, 1965, about 600 demonstrators led by the Southern Christian Leadership Conference and the Student Nonviolent Coordinating Committee set out to march from Selma to Montgomery to protest barriers to black voter registration and the fatal shooting of a demonstrator, Jimmy Lee Jackson, by a state trooper in Marion some three weeks before.

State troopers and sheriffs deputies halted the march, used tear gas and beat the marchers, sending some to the hospital.

The violence drew national attention to the fierce resistance to black civil rights in Alabama.

Two weeks after Bloody Sunday, marchers led by Martin Luther King Jr., started out again, this time with the authorization of a federal court order and the protection of National Guard troops.

They reached the State Capitol on March 25, and King spoke to a crowd estimated at 25,000 that day.

The march and Bloody Sunday galvanized support for the Voting Rights Act, passed by Congress later that year. It led to the end of schemes used to keep blacks from registering.

The president said Americans don't place enough value on what the Bloody Sunday marchers achieved because too few participate in elections.

"Fifty years ago, registering to vote here in Selma and much of the South meant guessing the number of jellybeans in a jar, the number of bubbles on a bar of soap," Obama said. "It meant risking your dignity, and sometimes, your life.

'What's our excuse today for not voting? How do we so casually discard the right for which so many fought?"

The president said Bloody Sunday was one of the events that set the nation's destiny, likening it to Concord, Lexington, Appomattox and Gettysburg.

"In one afternoon 50 years ago, so much of our turbulent history -- the stain of slavery and anguish of civil war; the yoke of segregation and tyranny of Jim Crow; the death of four little girls in Birmingham; and the dream of a Baptist preacher -- all that history met on this bridge.

"It was not a clash of armies, but a clash of wills; a contest to determine the true meaning of America."

The annual Bloody Sunday commemorative bridge crossing is Sunday, part of a long slate of events that will include marchers making the 54-mile trek to Montgomery. They are scheduled to reach the State Capitol on Friday.