Barack Obama made a surprise appearance at the Democratic National Convention Wednesday night, after former U.S. president Bill Clinton gave him a rousing endorsement.

"Just in case you were wondering, I think president Bill Clinton reminded us of what it's like when you've got a president who actually puts people first," said Obama. "Thank you, president Clinton."

Leading up to Clinton's speech, there was a great deal of speculation of what he would say, as he had seemed tepid in his support of Obama after the primaries. But Clinton said everything that Democrats could have hoped he would to restore party unity.

"Everything I've learned in my eight years as president and the work I've done since, in America and across the globe, has convinced me that Barack Obama is the man for this job," he said.

Some reports indicate Clinton felt the Obama camp hadn't been respectful of his legacy as the last Democratic president.

Clinton, who was an inexperienced and young politician when he was elected president in 1992, noted the similarities between himself and Obama. He described them both as forces of change.

"Obama is on the right side of history," Clinton said.

He also took numerous shots at the Republicans, noting how he gave them record surpluses when he left office and under George W. Bush, the U.S. now has a record debt.

"The Republicans will elect a good man," Clinton said of Sen. John McCain, before criticizing him as being "more of the same."

There was an enthusiastic crowd response for the last Democratic president of the United States.

"Sit down," Clinton said with a laugh, trying to calm the floor of Denver's Pepsi Centre.

Michelle Obama stood up clapping several times during the speech as Clinton praised her husband as a future president.

Clinton also praised Obama's pick of Sen. Joe Biden as his running mate, calling it a "home run." Biden is a veteran of the senate and was first elected at the age of 29. He was 30 when sworn in -- the lowest age possible under U.S. law.

"I love Joe Biden, and America will too," he said.

Biden takes stage

Addressing the crowd after Bill Clinton, Biden spoke of his working-class background, comparing his own life to Obama's humble beginnings.

Biden, who ran against Obama during the primary races, said the two men share the same values and belief in the American dream.

"You can learn an awful lot about a man campaigning with him, debating him, and seeing how he reacts under pressure. You learn about the strength of his mind. But even more importantly, you learn about the quality of his heart," said Biden.

"I watched how he touched people, how he inspired them, and I realized he has tapped into the oldest American belief of all: we don't have to accept a situation we cannot bear. We have the power to change it. And change it is exactly what Barack Obama will do."

Biden repeatedly echoed Obama's trademark theme of change, using it to attack McCain, who he called a friend and referred to by his first name.

"As oil companies post the biggest profits in history, nearly half a trillion dollars in the last five years, John wants to give them another $4 billion in tax breaks. That's not change -- that's the same," he said.

Obama makes history

Sen. Hillary Clinton stepped into the state-by-state presidential roll call of the Democratic National Convention earlier Wednesday to ask to suspend the roll call and officially back Obama by acclaimation.

"Let's declare together in one voice right here right now that Barack Obama is our candidate and he will be our next president," she said to cheers.

Obama is the first black American to win the presidential nomination from a major party.

Hillary Clinton's move made his nomination unanimous. It was the result of agreement between the two campaigns to help unify the party to face the Republicans.

Competing chants of "Yes, we can" and "Obama" thundered from the convention floor as Obama's victory became official.

Hillary Clinton told her delegates previously in the day that she was releasing them to vote for Barack Obama, one day after she gave a major speech endorsing her former rival.

She was formally nominated today as a way to honour her and soothe her supporters, many of whom are still bitter about her loss. Healing those wounds is important. Hillary Clinton had 18 million people vote for her, and she was dominant among women and blue-collar males -- two groups that still haven't warmed to Obama.

Bill Clinton's image as an elder statesman was damaged during the nomination fight because some of his remarks veered to the nasty as he campaigned for his wife.

Obama campaigned in Billings, Mont. on Tuesday and arrived in Denver today, before making his surprise appearance at the convention. He will deliver his prime-time nomination speech on Thursday before 80,000 people.

He does so as a new poll had him in a statistical tie with McCain, although another poll put him ahead in several critical swing states.