A central provision of President Barack Obama's healthcare reform law has taken effect, having survived Republicans' years-long effort to undermine it.
Markets run by federal and state governments opened for the millions of Americans seeking insurance plans.
As many as seven million Americans who do not already have health insurance are eligible to purchase coverage in the coming months.
Republican opposition to the law led to the government shutdown on Tuesday.
While Mr Obama and his supporters in the Democratic Party portray the law as a historic effort to extend healthcare coverage to the millions of Americans who lack it, the law's opponents say it amounts to an unprecedented intrusion into Americans' private lives.
The opening of the private health insurance marketplaces, or exchanges, was the culmination of more than three years of political combat in Washington over the Patient Protection and Affordable Care Act, signed into law by President Barack Obama in 2010 and known to both sides as Obamacare.
'Shutdown triggered'Republicans in Washington and in state capitals across the country have vehemently fought the law since its passage, with the US House of Representatives taking dozens of symbolic votes to repeal it or eliminate its funding.
A legal challenge led by Republican opponents of the law ended when the Supreme Court in June 2012 validated the law's keystone provision - a requirement that Americans not receiving health coverage from their employers or the government purchase individual plans or pay a fine.
That requirement, known as the individual mandate, takes effect in January.
The healthcare law was also a central issue in the 2012 presidential election. Republican candidate Mitt Romney vowed to press for its repeal. Mr Obama handily won re-election.
On Tuesday, the US government shut non-essential services, keeping more than 700,000 workers at home, when the Democrats who control the Senate refused to agree to Republican demands to repeal the health law or delay the individual mandate.
Both Mr Obama and congressional Democrats have said they will not negotiate on Obamacare, with Mr Obama on Monday evening accusing the Republicans of attempting to refight the last election.
In spite of the political turmoil in Washington, on Tuesday Americans were able to enrol in insurance plans offered by private companies and ranging in price and level of coverage through exchanges in every state.
Even supporters of the law anticipated that technical glitches and consumer confusion would slow adoption.
Sixteen states and Washington DC have opted to operate their own exchanges, while the exchanges in 34 states will be fully or partially run by the federal government.
'Major concession'Exchanges will also inform customers of tax subsidies available to ease the cost of the insurance. An estimated six million US citizens are expected to qualify.
Customers enrolling under Obamacare will not be covered by the insurance until 1 January however.
That corresponds with the start of the individual mandate, as well as consumer protections including a rule barring insurers from denying coverage to people with pre-existing health conditions.
Several Obamacare provisions had taken effect prior to Tuesday, including prescription drug discounts for pensioners, a rule allowing children to remain on their parents' insurance plans up to age 26, a rule barring insurers from denying coverage to children with pre-existing health conditions, and a ban on lifetime limits on health coverage.
Another major provision known as the employer mandate - which requires employers with at least 50 full-time workers to provide insurance or incur a $2,000 (￡1,320) per employee penalty - has been delayed until 2015.
That delay, announced by the Obama administration in July, has been seen as a concession to retailers and other businesses - and an acknowledgement that the health law was not fully ready for implementation.
