"And then there are those, like John McCain, who use their careers to promote change."
She also targeted Mr Obama's experience as a community organiser and remarks he made earlier this year when he spoke of "bitter" working-class people "clinging to guns or religion".
"I guess that a small-town mayor is sort of like a 'community organiser', except that you have actual responsibilities," she said.
"I might add that in small towns, we don't quite know what to make of a candidate who lavishes praise on working people when they are listening, and then talks about how bitterly they cling to their religion and guns when those people aren't listening."
Mrs Palin - who supports drilling for oil in Alaska's Arctic National Wildlife Refuge - said that while drilling "will not solve all of America's energy problems", that is "no excuse to do nothing at all".
Democrats under fire
Former Governors Mitt Romney and Mike Huckabee opened the night by hailing Mr McCain and attacking the Democrats.
Mr Romney, a one-time rival of Mr McCain for the Republican nomination, used his speech to hammer the Democrats over their "liberal" agenda.
Romney urges vote for McCain
"We have a prescription for every American who wants change in Washington - throw out the big government liberals and elect John McCain," the former Massachusetts governor said.
He also lauded Mr McCain's national security credentials, saying he was the presidential contender who would defeat "evil" radical Islam.
Mr Huckabee, also a former rival of Mr McCain, joked that he had hoped to be giving the speech on Thursday night - when Mr McCain will accept the party's nomination to run for president in November's election.
But, he said, he was delighted to be speaking for his second choice, Mr McCain - "a man with the character and stubborn kind of integrity that we need in a president".
He defended Mrs Palin against criticism from the media, saying its coverage had been "tackier than a costume change at a Madonna concert", and attacked the Democrats' vice-presidential candidate Joe Biden.
You need to face your enemy in order to defeat them. John McCain will face this threat and bring victory to this country
"I am so tired of hearing about her lack of experience. She got more votes running for mayor of Wasilla, Alaska, than Joe Biden got running for president of the United States," he said, referring to Mr Biden's performance in the Democratic primaries.
Former New York City Mayor Rudy Giuliani followed Mr Huckabee on stage, calling the 2008 presidential election a "turning point" for the people of the US.
He charged the Democrats with being in denial about the threat from terrorism and said Mr McCain had the foreign policy, national security and leadership experience that counted.
"The choice in this election comes down to substance over style," he said. "John has been tested. Barack Obama has not. Tough times require strong leadership, and this is no time for on the job training."
The Alaska governor's speech comes amid scrutiny of her record and after two days dominated by the news her daughter Bristol, 17, is pregnant.
Mrs Palin and her family, including Bristol and her boyfriend, greeted Mr McCain at the airport as he arrived in Minnesota on Wednesday.
Bristol Palin and boyfriend greet John McCain at Minneapolis-St Paul airport
Ahead of her address, senior McCain campaign adviser Steve Schmidt issued a statement saying that media questions over how thoroughly Mrs Palin was vetted should end.
It has also been revealed that an attorney has been hired to represent Mrs Palin in an Alaska state ethics investigation involving alleged abuse of power.
Mrs Palin told US network CNBC she had "nothing to hide". Her deposition is expected to be scheduled soon.
There have also been reports that Mrs Palin sought special financial favours for her city and state - something the McCain campaign is against.
She was elected governor of Alaska in 2006 and before that was mayor of the small town of Wasilla, Alaska.
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