"I want to go up to the closest white person and say, 'You can't understand this, it's a black thing,' and then slap him, just for my mental health," Charles Barron, a member of the New York City Council, told the crowd.
The demonstrators, numbering about 2,000 to 3,000, came from all parts of the United States, many traveling by bus from as far away as Texas.
With the U.S. Capitol in the background, they chanted "Black power! Reparations!" and "Start the Revolution!"
"Apologize White America," said a sign carried by one demonstrator.
Barron, a self-proclaimed "elected revolutionary," said if the government did not act swiftly he personally would storm the Treasury Department and take the money for reparations.
Nation of Islam leader Louis Farrakhan told the crowd, "America owes the black people a lot for what they've endured. We cannot settle for some little jive token. We need millions of acres of land that black people can build."
"We're not begging white people," said Farrakhan, one of several speakers at a rally organizers billed as "Millions for Reparations." "We are just demanding what is justly ours."
While Farrakhan and Rep. John Conyers, D-Mich., attended, many major names in the black civil rights movement were absent, including the Rev. Jesse Jackson and the Rev. Al Sharpton.
"The people on the ground are the ones we want to give exposure to," said one speaker, Hannibal Afrik, 68, of Port Gibson, Miss. "If it's grounded in the people, it will be victorious."
Conyers, who for 13 years has been proposing a commission to study the institution of slavery, urged the crowd to pressure Congress.
"Only the Congress can do what we want done," Conyers said. Lawmakers are now on their summer break until September.
The reparations movement has gained momentum in the past year.
Earlier this year, a group of slave descendants sued three companies, claiming the companies - or their corporate predecessors - unjustly profited from slavery.
The Reparations Coordinating Committee, which includes many prominent attorneys and scholars, is working on a separate lawsuit against the federal government.
Those at the rally said it was time for action.
"They owe us. I want justice," said Antoinette Harrell-Miller, who drove 19 hours from New Orleans with her husband Dennis to attend. "They built this country off the free labor of our ancestors."
Jaki Mungai of Philadelphia called the rally "a dream that's starting to come true."
"Every other group has been compensated for the wrongs done against them. Africans in America - we are the only ones who haven't," said Mungai, who heard about the rally on the radio and decided to join in.
Ken McDouall of Durham, N.C., one of a handful of whites at the rally, said the reparations issue "cuts to the core of the history of injustice of America."
"America likes to pretend there are equal rights for everyone but look at the history of black people," McDouall said.