US President Barack Obama says his embattled signature healthcare programme is "here to stay" after more than 7 million Americans signed up.
He told reporters the law, nicknamed Obamacare, represented progress and benefitted the economy.
Mr Obama also lashed out at repeated conservative efforts to repeal or defund the law, calling it "troubling".
There would be additional challenges to implementing the law, he said, but it was "working".
An estimated 7.1 million Americans signed up for coverage to avoid penalties prior to Monday's deadline for doing so, exceeding initial projections.
"This law has made our healthcare system a lot better," Mr Obama said at the White House on Tuesday, adding that it would lower overall healthcare costs.
Though the reform had at times been "contentious and confusing", he said, "that's part of what change looks like in a democracy.
"Change is hard. Fixing what is broken is hard."
The Democrat leader has battled political opposition to Obamacare - officially the Patient Protection and Affordable Care Act - for years.
Republicans - who see it as an inappropriate government intrusion into the healthcare industry and an affront to personal liberty - are not expected to relent in their opposition to the law.
The law also remains controversial among the American public, as some people have seen their insurance costs rise or their old plans cancelled, and others object to having to purchase insurance at all.
The Republicans hope to tap into voter discontent with the law in November's midterm elections.
The 2010 law is intended to extend health insurance to the roughly 48 million Americans who do not receive it through their employers, the government, or a privately purchased plan.
It also aims to slow the growth in the cost of healthcare and requires private plans to meet a certain level of coverage.
The federal Healthcare.gov and similar marketplace websites run by some states are a key element of the law, providing a clearing-house for people to purchase health insurance for themselves and their families, often with generous subsidies.
Tax penalties ahead
In addition to the more than seven million people signed up for private plans through the marketplaces, between 4.7-6.5 million have gained insurance through an expansion of government-run health programmes for the poor, and 2.5-3.1 million men and women under the age of 26 have signed up for newly available coverage on their parents' insurance plans, according to tracking website acasignups.net.
From now on, most people who are not covered by some form of health insurance will face a tax penalty next year.
Final enrolment figures for 2014 will not be available for some time as the Obama administration has allowed those who had technical difficulties to continue to sign-up. Millions could be eligible for such extensions.
And it is unclear yet how many people who have enrolled in insurance through the websites have paid their premiums - a necessary step before the coverage can begin.
Healthcare.gov relaunched on Tuesday for special enrolment and for people who had been blocked from signing up because of technical problems. Some state-run sites were also giving residents grace periods.
During the final push, the Obama administration made a special effort to get younger Americans, who are crucial to the law's success, to sign up.
Celebrities who are supporters of Mr Obama tweeted about the programme and Vice-President Joe Biden joined the Rachel Ray chat show on Monday to talk about healthcare.