Thousands of demonstrators marched through this small Louisiana town Thursday protesting racial injustice after stiff criminal charges were brought against a group of black students who beat up a white student in a school fight.

The case, which has become a potent example for civil rights leaders of widespread inequality in the US criminal justice system, comes after a series of race incidents at the high school in Jena.

Racial tensions first erupted after a black student tried to cross an invisible color line and sit under the schoolyard's "white tree" to be greeted the next morning by nooses hanging from the tree.

Several fights broke out on and off campus afterwards and a fire was set in the school after the authorities refused to expel the three white students who hung up the nooses -- long a symbol of anti-black violence in the south -- calling it an "adolescent prank."

In most of the incidents, the white students escaped any criminal charges. But tensions flared to a new height when after a December schoolyard brawl six black students were charged with attempted murder.

While the charges against the Jena Six, as they have become known, were eventually reduced, the students still face stiff penalties.

Civil rights leader Rev. Jesse Jackson likened Thursday's protest to the great civil rights marches in America in the 1950s.

"Just as Little Rock defined desegregation ... today we march, fighting for criminal justice equality," Jackson told the crowd.

"The Department of Justice in Washington's gone silent," he said. "We are intent to have hearings of the matter of criminal justice in Jena because there is a Jena in every town, a Jena in every state."

Critics here accuse the local district attorney of racism for failing to hand out equal punishment to the white students who started fights with their black peers.

"It's not equal," said Tina Jones, the mother of one of the Jena Six.

"The black people get the harsher extent of the law whereas white people get a slap on the wrist," she told CNN.

"I hope the DA (district attorney) will wake up and realize that he's doing the wrong thing and to release these kids and let them go."

But LaSalle Parish district attorney Reed Walters told a press conference with the white victim of the school beating, Justin Barker, standing silently behind him, that the case "is not and never has been about race."

He denounced the students who hung the nooses from a tree last September but said he was unable to prosecute them because it did not qualify as a hate-crime -- a conclusion also reached by the region's federal prosecutor.

"This was an awful act," Walters said. "It was not a prank but a vicious and crude statement ... The people who did it should be ashamed of themselves and mortified at the havoc they have unleashed on this community."

President George W. Bush said on Thursday: "The events in Louisiana have saddened me. And I understand the emotions.

"The Justice Department and the FBI are monitoring the situation down there. And all of us in America want there to be, you know, fairness when it comes to justice."

Bush insisted the Republican party had a "good record" on race relations, but Democratic presidential hopeful Barack Obama, who is black, seized on the protest to call for change.

"When a noose hangs from a schoolyard tree in the 21st century and young men are treated in a way that is not equal nor just, it is not just an offense to the people of Jena or to the African-American community, it is an offense to the ideals we hold as Americans," he said in a statement.

Uneven sentencing for blacks and whites is common across the country, according to a report by the New York-based Urban League.

African-American men are three times more likely than white men to face jail once they have been arrested:24.4 percent of blacks arrested in the United States in 2005 ended up in jail compared with 8.3 percent of white men.

They also receive jail sentences that are on average 15 percent longer than whites convicted of the same crime.

The biggest disparity is among men convicted of aggravated assault:black men were sentenced to an average of 48 months in jail, which is 33 percent longer than the average sentence of 36 months received by white men, according to the League's annual State of Black America report.