"This bill is the greatest threat to freedom that I have seen," House Republican leader John Boehner of Ohio told the crowd gathered on the lawn near the West Front of the Capitol.
The protest attracted many of the so-called Tea Party demonstrators angry with increased spending and an expanded government role under the Obama administration. Their signs ranged from the harsh, "Waterboard Congress," to an echo of the rallying cry at August town halls with lawmakers, "Vote no to government-run health care."
One protester carried a placard reading, "Bury Obamacare with Kennedy," a reference to Sen. Edward Kennedy, D-Mass., who died of brain cancer this past summer.
The demonstrators came to Washington by plane, bus and other means to send a message to House Speaker Nancy Pelosi, D-Calif., and Senate Majority Leader Harry Reid, D-Nev., as they press ahead with health care legislation.
"It's upsetting because of where we've come from to where we are today, to see our freedom eroding," said Ben Fourman, 72, of Farmingdale, N.J. The retiree wore a T-shirt that said, "Liberty equals limited government."
Deborah Stevenson, 58, of Southbury, Conn., boarded a bus Thursday at 3:30 a.m. EST to make the noon rally. "Congress and the entire government need to pay attention to the Constitution and stop passing unconstitutional law," she said.
Ken Klyberg, of Satellite Beach, Fla., flew to Washington with his wife Misty. "Common sense is lacking in the decisions that are being made in Washington," he complained.
Rep. Steve King, R-Iowa, told the crowd, "We're not going to leave this Hill until we kill this bill."
Democrats hope to pass the far-reaching legislation on Saturday and gained the backing Thursday of the American Medical Association and the AARP, the powerful seniors lobby.
Inside the Capitol, legislative action continued in the House and the Senate.
The White House downplayed the rally. Obama spokesman Robert Gibbs criticized a Republican reform plan, saying it wouldn't reduce government cost of health care and includes "old ideas."
"There's a rally going on without a solution on their side," Gibbs said.
Associated Press Writer Ann Sanner contributed to this report.