Two Democratic former presidents, Bill Clinton and Jimmy Carter, and former Secretary of State Colin Powell, a Republican, endorsed Mr Biden.
Mr Clinton said President Donald Trump had brought “chaos” to the Oval Office.
Mr Trump trails Mr Biden in opinion polls ahead of November’s election.
The convention is largely virtual, amid the coronavirus pandemic, and it is unclear whether a format of pre-recorded speeches and no live audience will generate the same levels of enthusiasm as the traditional party gatherings. Next week’s Republican convention will also be mostly online.
Biden, the former vice-president under President Barack Obama, became his party’s nominee on Tuesday night in a pre-recorded roll call vote from delegates in all 50 states.
This is Mr Biden’s third White House bid, having formerly run in 1988 and 2008. The 77-year-old’s campaign appeared to be in danger of collapse back in February this year.
On the second night of the party convention on Tuesday, with the theme “leadership matters,” Mr Clinton delivered the key address.
“Donald Trump says we’re leading the world,” Mr Clinton said in his five-minute message pre-recorded from his home in Chappaqua, New York. “Well, we are the only major industrial economy to have its unemployment rate triple.
“At a time like this, the Oval Office should be a command centre. Instead, it’s a storm centre. There’s only chaos.”
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