WASHINGTON - President Barack Obama demanded action now from Congress to fix a health system reduced to "breaking point," vowing to end the moral taint that deprives millions of Americans of treatment.

In a bold bid to assert leadership as a fractious debate rattles his young administration, Obama strode late Wednesday into a rare joint address to Congress and rejected some conservative attacks on his plan as lies.

"We did not come here to fear the future, we came here to shape it," said Obama, reprising the reformist zeal that powered his triumphant 2008 election campaign, but which has been sullied by months in Washington's partisan swamp.

"I still believe we can act when it is hard," he said.

"I am not the first President to take up this cause, but I am determined to be the last," he added to a huge ovation from lawmakers.

After a summer of political fury, Obama offered the most detailed outline of his reform plan yet, warned more Americans would die if Congress did nothing, and told Republicans not to waste his time by trying to kill the plan.

"The time for bickering is over. The time for games has passed. Now is the season for action... now is the time to deliver on health care," said Obama, in a speech punctuated by 27 standing ovations.

"Our collective failure to meet this challenge -- year after year, decade after decade, has led us to the breaking point," he said promising health care for the first time to 47 million uninsured Americans.

Though Obama did offer an olive branch to Republicans, including a promise to tackle liability suits that make insurance unaffordable for some doctors, latent political tensions spilled over.

Congressman Joe Wilson of South Carolina shattered protocol and shouted out "you lie!" when Obama said his plan would not give health coverage to undocumented immigrants, earning him a furious look from House speaker Nancy Pelosi.

Many other Republicans sat stone faced through much of his address, but laughed derisively when Obama said "there remain some significant details to be ironed out" in crafting legislation.

The president, who has seen his approval ratings tumble, took aim at some of the most explosive charges thrown at his plan by conservative critics.

He said a claim he wanted to set up a "death panel" to ration end of life care was "laughable ... cynical and irresponsible."

"It's a lie, plain and simple," he said.

In one moving passage, Obama remembered his late political mentor senator Edward Kennedy, who died last month, but not before writing a last letter to the president that he ordered delivered after his death.

"'What we face," he wrote, 'is above all a moral issue; at stake are not just the details of policy, but fundamental principles of social justice and the character of our country'," Obama said.

As Obama spoke, Kennedy's widow Vicki looked on from the First Lady's box. Vice President Joe Biden wiped away a tear.

The address to Congress laid out Obama's most detailed summary yet of his health care reform drive, as he directly argued for the concept of including a government-run component in the plan -- a move beloved by his supporters but which opponents say a step towards a state-run national health service.

But he also signaled the plan was not a deal breaker, as intense negotiations go on in Congress to find a compromise.

Sharpening his rhetoric, Obama warned his "Republican friends" would not hold up his plan just to appear bipartisan, in a speech launching a new push on health care, which includes a rally in Minnesota on Saturday.

In the official Republican response, representative Charles Boustany called for the president to "start over".

Boustany, a heart surgeon by profession, slammed the Democrat's plan in Congress by charging "it creates 53 new government bureaucracies, adds hundreds of billions to our national debt, and raises taxes on job creators by 600 billion dollars."