"Not on our watch," the crowd chanted, as a parade of speakers took turns on the stage - exhorting the world to speak out against the genocide in the Darfur region of Sudan, and calling on the Bush administration and Congress to use their political clout to stop it.
The rally's organizers – a diverse coalition including celebrities Jewish groups, Sudanese immigrants, black Americans, evangelical Christians, Catholics, and gay rights groups – say they bused people in from 41 states and estimated the crowd at between 10,000 and 15,000.
The police made no official estimate of the size of the crowd.
"The personal motivation for a lot of us is the Holocaust," said Boston-based Rabbi Or Rose of Jewish Seminarians for Justice. "Given our history and experience, we feel an obligation to stand up and speak out."
Holocaust survivor and author Elie Wiesel told the crowd, "for the sake of our humanity, save Darfur," CBSNews.com's Amy Sara Clark reports.
Wiesel said while families are being uprooted and starved and children tormented and slaughtered, the world is "indifferent to their plight."
He said he won't remain silent because "silence helps the killer, never his victims."
The D.C. rally, which was peaceful with no arrests, was one of several events in U.S. cities over the weekend over what the United Nations has termed the world's worst humanitarian disaster.
"It is the social responsible, good conscience thing to do," said Ron Fisher, who took a pre-dawn bus from Cleveland with his 15-year-old daughter Jordyn to attend the demonstration. "It's an opportunity to show my daughter what people do when they care about something."
CBSNews.com's Clark reported from the rally that marchers wore orange stickers with numbers, each signifying a Darfur victim. Some carried banners and signs with slogans including "Never again" and "Save Darfur."
The event attracted high-profile speakers from the worlds of screen, athletics, religion and politics: actor George Clooney, just back from a trip to Africa; Sen. Barack Obama, D-Ill.; House Democratic leader Nancy Pelosi of California; Nobel Peace Prize winner Elie Wiesel; Olympic speedskating champion Joey Cheek, who donated his bonus money to projects in war-orn Darfur; and Roman Catholic Cardinal Theodore McCarrick, archbishop of Washington.
"If we care, the world will care," Obama said. "If we act, then the world will follow." Pelosi said Democrats for once agree with Mr. Bush: "This genocide must stop."