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Dressed casually in khakis and a jacket, Obama told a rally of some 12,000 people in Denver, Colorado the former Massachusetts governor was untruthful during their 90-minute debate, which most observers said the Republican won.
"When I got onto the stage, I met this very spirited fellow who claimed to be Mitt Romney," Obama said.
"But it couldn't have been Mitt Romney, because the real Mitt Romney has been running around the country for the last year promising $5 trillion in tax cuts that favour the wealthy. The fellow on stage last night said he didn't know anything about that."
Obama's campaign has pressed Romney for months over his support for extending Bush-era tax cuts for the wealthy, which the nonpartisan Tax Policy Center estimated could cost $5 trillion over 10 years. Romney rejected that figure at the debate and insisted his plans would not increase the deficit.
The Democratic president was criticised for not pushing back aggressively against his rival during the debate. He adopted an assertive posture on Thursday, however, taking Romney to task on everything from education policy to outsourcing.
"The real Mitt Romney said we don't need any more teachers in our classrooms. But ... the fellow on stage last night he loves teachers - can't get enough of them," Obama said.
"The Mitt Romney we all know invested in companies that were called 'pioneers' of outsourcing jobs to other countries. But the guy on stage last night, he said he doesn't even know that there are such laws that encourage outsourcing."
Obama's senior campaign strategist, David Axelrod, said the campaign would adjust its strategy as a result of the debate.
"We are going to take a hard look at this and we are going to have to make some adjustments as to where to draw the lines n these debates and how to use our time," Axelrod told reporters.
"It's like ... playoffs in sports, you evaluate after every contest and you make adjustments and I am sure that we will make adjustments. I don't see us adding huge amounts of additional prep time. I think there are some strategic adjustments that have to be made and we'll make them."
Obama also zeroed in on Romney's comments about the deficit, particularly his pledge to cut funding for public television and a popular children's show character known as Big Bird.
"When (Romney) was asked what he'd actually do to cut the deficit and reduce spending, he said he'd eliminate funding for public television. That was his answer," Obama said.
"I mean, thank goodness somebody is finally getting tough on Big Bird! It's about time. We didn't know that Big Bird was driving the federal deficit."