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The line of protesters completely filled Pennsylvania Avenue for blocks, all the way to the capitol, according to the D.C. 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.
Others were waving US flags and holding signs reading "Go Green Recycle Congress" and "I'm Not Your ATM." Some men are dressed in colonial costumes. Police on motorcycles and horses watched as the marchers passed.
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 organisation led by former House Majority Leader Dick Armey, organised several groups from across the country for what they billed as a "March on Washington."
Organisers 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 US at risk.
Other sponsors of the rally include the Heartland Institute, Americans for Tax Reform and the Ayn Rand Center for Individuals Rights.