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The demonstrations came as the US president sought to boost support for his plans during a Minneapolis rally.
Addressing a crowd of 15,000, Mr Obama said he refused to accept no change on his top domestic priority.
He said he would not allow special interests to "use the same old tactics to keep things the way they are".
"I will not accept the status quo. Not this time. Not now," Mr Obama said in the democratic-leaning state, which has one of the country's smallest numbers of uninsured residents.
'Taxed to death'
But in Washington protesters attacked Mr Obama's administration for what it called out-of-control spending - on healthcare, the stimulus packages and the bailout of the banking and car industries.
Healthcare in the US costs $2.2tn a year, or 16% of the country's GDP - nearly double the OECD average.
The protesters insist that spending tax dollars on a government-run health insurance option will increase inflation and lead the country to economic ruin.
"Born free, taxed to death," one protester's sign read while another, held up by an immigrant from Ukraine, said: "I had enough of socialism in the USSR."
The march - co-ordinated by the conservative Freedomworks organisation which calls for lower taxes and smaller government - brought together protesters from across different states.
Days after urging Democrats and Republicans in Congress to work together, Mr Obama said his plan was open to ideas from across the political spectrum.
"If you come to me with a serious set of proposals, I will be there to listen. My door is always open," he said.
He warned, though, that he would not waste time with those who believe: "it's better politics to kill this plan than improve it".
It is estimated that some 46 million Americans do not have health insurance, and a further 25 million are thought to have inadequate insurance.
The healthcare plans currently being considered in Congress are all attempting to expand coverage, while also reforming the system to prevent spiralling costs.
The bill would expand coverage to 97% of Americans, at a cost of some $900bn (£540bn).