The line of protesters spread across Pennsylvania Avenue for blocks, all the way to the capitol, according to the Washington Homeland Security and Emergency Management Agency.
People were chanting "enough, enough" and "We the People." Others yelled "You lie, you lie!" and "Pelosi has to go," referring to California congresswoman Nancy Pelosi.
Demonstrators waved U.S. flags and held signs reading "Go Green Recycle Congress" and "I'm Not Your ATM." Men wore colonial costumes as they listened to speakers who warned of "judgment day" - Election Day 2010.
Richard Brigle, 57, a Vietnam War veteran and former Teamster, came from Michigan. He said health care needs to be reformed - but not according to President Barack Obama's plan.
"My grandkids are going to be paying for this. It's going to cost too much money that we don't have," he said while marching, bracing himself with a wooden cane as he walked.
FreedomWorks Foundation, a conservative organization led by former House of Representatives Majority Leader Dick Armey, organized several groups from across the country for what they billed as a "March on Washington."
Organizers say they built on momentum from the April "tea party" demonstrations held nationwide to protest tax policies, along with growing resentment over the economic stimulus packages and bank bailouts.
Many protesters said they paid their own way to the event - an ethic they believe should be applied to the government.
They say unchecked spending on things like a government-run health insurance option could increase inflation and lead to economic ruin.
Terri Hall, 45, of Florida, said she felt compelled to become political for the first time this year because she was upset by government spending.
"Our government has lost sight of the powers they were granted," she said. She added that the deficit spending was out of control, and said she thought it was putting the country at risk.
Anna Hayes, 58, a nurse from Fairfax County, stood on the Mall in 1981 for Reagan's inauguration. "The same people were celebrating freedom," she said. "The president was fighting for the people then. I remember those years very well and fondly."
Saying she was worried about "Obamacare," Hayes explained: "This is the first rally I've been to that demonstrates against something, the first in my life. I just couldn't stay home anymore."