Most of the demonstrators were dressed in black and some held banners reading "Free the Jena 6" and chanted "no justice, no peace, no racist police."
The protest follows an incident in August last year at Jena High School, north-west of New Orleans, when a black student sat under a tree that was a frequent gathering place for whites, the AP news agency reported.
The next day three nooses, widely seen as a symbol of the lynching of blacks in US history, were found hanging from the tree.
Three white students were later suspended over the incident but not criminally prosecuted.
Reed Walters, the LaSalle Parish district attorney, said state law did not cover such actions.
After several fights between students at the school, a white student was beaten unconscious and the six black students were charged with assault in connection with the case.
Charges against some of the black youths were later changed to attempted murder, drawing accusations they were being treated too harshly. The charges were later reduced.
For many black Americans the "Jena six" case represents a modern-day version of the incidents that surrounded the US civil rights movement in the 1960s, many of which occured in the region.
"This is the most blatant example of disparity in the justice system that we've seen," Sharpton said on television before arriving in Jena.
"We didn't bring race in it," he said. "Those that hung the nooses brought the race into it."
Sharpton said the rally could spark a civil rights movement for the new century that would challenge apparent disparities in the US justice system.
Walters, the district attorney prosecuting the black teenagers, has denied racism was a factor in bringing the charges.
"This is the most blatant example of disparity in the justice system that we've seen"
Reverend Al Sharpton, civil rights campaigner
"It is not and never has been about race," he said. "It is about finding justice for an innocent victim and holding people accountable for their actions."
Walters said incidents such as hanging nooses from trees were not covered under Louisiana law.
A report by the the New York-based Urban League says disparities in punishment between black and white people are common across the US.
Black men are sentenced to an average of 48 months in jail for aggravated assault, a third longer than the 36 months received by white men, the report says.