US Democratic presidential hopeful Barack Obama has introduced veteran Senator Joe Biden as his running mate at a rally in Springfield, Illinois.
Mr Obama hailed Mr Biden as a "man with a distinguished record and a fundamental decency".
Mr Obama confirmed his choice of running mate overnight on his website and with a text message after the news began to leak to the media.
The two men were making their first appearance following the announcement.
What the choice says about Obama
The Democratic campaign will be hoping Mr Biden's presence will reassure voters who are concerned about Mr Obama's relative inexperience, particularly in the international arena, says the BBC's Rachel Harvey at the rally.
Republican presidential hopeful John McCain's camp called the choice of Mr Biden an admission by Barack Obama that he was not ready to be president.
His spokesman also picked up on a slip of the tongue Mr Obama made on stage when he introduced his running mate as "the next president".
Hugs and cheers
At the place where he launched his presidential campaign a year and a half ago, Mr Obama outlined Mr Biden's accomplishments in the Senate, his blue-collar roots and - above all - his experience on foreign policy.
OFFICE OF THE VICE-PRESIDENT
"He's an expert on foreign policy whose heart and values are rooted firmly in the middle class," Mr Obama said.
He also emphasised Mr Biden's drive for change, despite his 30 years spent in the Capitol.
"For decades, he has brought change to Washington, but Washington hasn't changed him," Mr Obama said.
He recounted the personal tragedy that struck Mr Biden more than 30 years ago, within days of his election to the Senate, when his first wife and their daughter were killed in a car accident.
After being introduced, a shirt-sleeved Mr Biden ran on to the stage and was embraced by Mr Obama to cheers from the crowd.
In his speech, Mr Biden referred to his own short-lived bid for the White House against Mr Obama for the 2008 nomination, before dropping out in January:
"You learn about a man when you debate with him, you see how he thinks. Barack Obama has the vision and courage to make this a better place. He is a clear-eyed pragmatist who will get the job done."
At one point, Mr Biden garbled Mr Obama's name, calling him "Barack America". The crowd yelled back "Obama".
Veteran politician
Mr Biden, a 65-year-old veteran lawmaker, is highly respected on foreign policy and is a six-term senator who serves on the powerful Foreign Affairs Select Committee.
HAVE YOUR SAY
He has represented the state of Delaware in the US Senate since 1972.
Crucially, Senator Biden appeals to working-class Americans and was born in Pennsylvania, a key swing state in this election, our correspondent says.
Hillary Clinton, the former first lady who narrowly lost to Mr Obama during the tense battle for the Democratic nomination, issued a statement calling Mr Biden "an exceptionally strong, experienced leader and devoted public servant".
John McCain has reportedly not yet settled on a running mate.
Mr McCain's spokesman, Ben Porritt, suggested that Mr Obama's slip in describing his running mate as "the next president" reflected on his own inexperience.
"Barack Obama sounded as though he turned over the top spot on the ticket today to his new mentor..." he said in a statement.
"The reality is that nothing has changed since Joe Biden first made his assessment that Barack Obama is not ready to lead," Mr McCain's spokesman said.
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