Former US President Bill Clinton has delivered a prime-time defence of Barack Obama, nominating the president for a second term in the White House.

His 50-minute speech at the Democratic convention in Charlotte, North Carolina, was strongly critical of Republican economic plans.

He launched a full-throated defence of Mr Obama's policies, saying his economic policies were working.

Mr Obama will take on Republican Mitt Romney in November's election.

Bill Clinton's speech is being seen as the high point of a revitalised relationship between the two presidents and as an attempt to boost Mr Obama's appeal with white working-class voters.

Polls show these traditional Democratic voters are wary of Mr Obama, but Mr Clinton has a strong record in winning their support.

Mr Clinton told the crowd that they would "decide what kind of country you want to live in".

"If you want a 'you're on your own, winner take all society' you should support the Republican ticket," he said. "If you want a country of shared opportunities and shared responsibilities - a 'we're all in it together' society - you should vote for Barack Obama and Joe Biden."

Mr Clinton accused Republicans of having blocked further progress on the economic recovery.

"In order to look like an acceptable, moderate alternative to President Obama, they couldn't say much about the ideas they have offered over the last two years," he said, referring to the Republican convention in Florida a week ago.

Reminding the crowd that Senate Republican leader Mitch McConnell had revealed that their number one priority was to get Mr Obama out of office, he declared: "We're going to keep President Obama on the job."

Mr Clinton argued that Mr Obama's economic policies on taking office had prevented further collapse and begun the recovery, but said he knew that many Americans were still struggling.

He compared Mr Obama's experience to his own first term in office, when "our policies were working and the economy was growing but most people didn't feel it yet".

"No president. No president - not me, not any of my predecessors - no-one could have fully repaired all the damage that he found in just four years," he said.

"But he has laid the foundations for a new, modern, successful economy of shared prosperity. And if you will renew the president's contract you will feel it. You will feel it."

Mr Clinton criticised Republican vice-presidential candidate Paul Ryan, who he said misrepresented Mr Obama's Medicare policy at last week's Republican convention.

He argued that Mr Ryan had made the same amount in cuts as part of his plan for government-sponsored healthcare for the elderly.

"It takes some brass to attack a guy for doing what you did," Mr Clinton.

He also countered a Republican ad that Mr Obama had weakened the work requirement for welfare, which Mr Clinton signed into law.

"When some Republican governors asked to try new ways to put people on welfare back to work, the Obama administration said they would only do it if they had a credible plan to increase employment by 20%," Mr Clinton said, adding that the Republican charge was "just not true".

After the former president finished a lengthy and partially ad-libbed speech, Mr Obama joined him on stage.

The pair have previously sparred, most notably during the 2008 primaries when Mr Clinton supported his wife Hillary's bid for the nomination, and they are known not have a close personal bond.