Before Obama signed the sweeping bill, guests for the signing ceremony took pictures with cameras and cellphones.
Among the guests was Rep. Bart Stupak, D-Mich., who led a bloc of anti-abortion Democrats large enough to have blocked the bill's passage. Obama said he would issue an executive order to ensure existing limits on federal funding for abortions remain intact, clearing way for the bill's passage.
Sounding like a raucous rally instead of a subdued signing ceremony, the standing-room-only audience clapped, cheered and chanted "Fired up ... ready to go," a slogan from Obama's presidential campaign.
"Ladies and gentlemen, to state the obvious, this is a historic day," Vice President Joe Biden said. "Mr. President, you're the guy who made it happen. ... You've done what generations of great men and women ... have tried to do" beginning with President Theodore Roosevelt.
Interrupted several times by lengthy rounds of applause, Obama said, "It is fitting that Congress pass this historic legislation this week (that) ... marks a new season in America."
He said he expected the Senate to pass a package of fixes to the legislation soon and people in the audience applauded and pumped their fists in the air.
The legislation had seemed virtually dead as recently as a month ago.
"The bill I'm signing will set in motion reforms that generations of Americans have fought for and marched for and hungered to see," Obama said, noting it will take four years for the the bill to be implemented fully.
"But a host of desperately needed changes will take effect right away," he said, such as health insurance reforms that bar insurance companies from dropping people's coverage when they become sick and from imposing lifetime limits; allowing uninsured Americans to have access to affordable insurance through an exchange program; requiring new insurance plans to offer free preventive care, and allowing young adults to remain on their parents' policies until age 26.
Seniors, he said, will get financial help to fill in the so-called "doughnut hole" in prescription drug coverage. And, contrary to some comments, "these reforms will not cut your guaranteed benefits," he said.
At the Interior Department, Obama thanked supporters, both those in person and people following either on the Web or television, for their tireless work to pass healthcare reform.
"When the opposition said it wasn't the right time, you didn't want to wait another year, another decade, another generation for reform," he said.
To opponents who called healthcare reform "Armageddon," Obama suggested waiting a month to see if the sky would fall.
But the work isn't done, Obama said. Passage of healthcare reform "doesn't erase the challenges we face," such as rebuilding the economy, spurring hiring, improving schools, reducing dependency on foreign oil," he said.