Saturday's "March on Washington" brought together several groups organised by the Freedom Works Foundation, a conservative organisation.
Obama on Thursday urged congress to agree to sweeping changes to the country's healthcare system, to make services more accessible to Americans.
The president said his plan would meet three basic goals: provide more security and stability to those who have health insurance, provide insurance to those who do not have it and slow the growth of healthcare costs.
However, there is vociferous opposition to the plan from Republican and conservative groups.
They allege that Obama's package amounts to a "socialist takeover" of the US healthcare system, and that spending $1 trillion on the overhaul is irresponsible at a time when the US is struggling under its highest-ever budget deficit.
Monica Villamizar, Al Jazeera's correspondent in Washington DC, said: "We are hearing that about 300,000 people are expected to show up.
"They say that congressmen are not hearing what the people want.
"These are radical, conservative people, but they are saying that they are not getting their voices heard."
Villamizar said that as well as state spending and Obama's healthcare proposal the demonstrators also disagreed with other Democrat policies.
"They are strongly against an open border for immigrants ... redistribution of wealth - basically higher taxes and a lot of the fiscal policies of this government - and a lot of the social liberal policies of this government, regarding abortion and homosexual marriages."
Many at the protest said that increased government spending could augment inflation and further deteriorate the economy, already suffering from its worst recession in several decades and the cost of state bailouts for the financial sector.
Norman Kennedy, a 64-year-old protester, said he wanted to send a message to federal lawmakers that America is "deeply in debt".
He said that although he would be happy for everyone to have free healthcare there was no money to pay for it.
"I want to see fiscal responsibility and if that means changing congress that will be a means to that end," he said, referring to Congressional elections in 2010.