Biden took centre stage on opening night of the Democratic National Convention on Monday, drawing an extended standing ovation from the party faithful, who have swiftly moved on without him.

Wiping away tears after being introduced by his daughter Ashley and waving to the crowd that held signs saying, "We (heart) Biden," a smiling Biden said: "I love you."

"Are you ready to vote for freedom? Are you ready to vote for democracy and for America? Let me ask you, are you ready to elect Kamala Harris and Tim Walz?" Biden said.

Biden's address in Chicago kicked off a four-day event fuelled by enthusiasm for Harris and relief that Biden abandoned his own re-election bid and endorsed her to replace him.

The president's reluctant decision to step aside on July 21 came after heavy pressure from party leaders who worried the 81-year-old incumbent was too old to win or serve another four years.

In his speech, Biden touted his administration's accomplishments - boosting the US economy and strengthening US alliances abroad - and made the case for Americans to elect Harris as his White House successor.

He condemned the racism and white supremacy that continue to plague America, saying they had no place in the nation.

Earlier in the evening, Harris has made a surprise appearance drawing cheers from the crowd as she vowed to defeat Republican rival Donald Trump in the US election.

"Let us fight for the ideals we hold dear and let us always remember, when we fight we win!" Harris, the US vice president, said in brief remarks that drew roars from the crowd.

Harris, 59, will formally accept the nomination on Thursday.

With AAP.