Chicago, Illinois - An emotional Joe Biden received a huge ovation as he gave a farewell speech at the Democratic National Convention in Chicago on Monday, after Kamala Harris made a surprise appearance to thank him for being an "incredible" president.

"I love you," the tearful 81-year-old president told the crowd to deafening cheers, less than a month after dropping out of the White House race against Donald Trump and passing the torch to his vice president.

Biden took to the stage to chants of "We love you Joe" from the same party which had pushed him out over concerns about his age, in one of the biggest political shocks in US political history.

"We both know we have more to do, but we're moving in the right direction," he said, referring to himself and the 59-year-old Harris, who, in the space of a few weeks, has turned the presidential race upside down.

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America's first female, Black, and South Asian vice president has restored hope to the Democratic Party and wiped out Republican rival Trump's lead in the polls.

She heaped lavish tribute to Biden on Monday, even breaking with tradition to speak on the first night of the convention in praise of her boss.

"I want to kick us off by celebrating our incredible President Joe Biden," said Harris, who was wearing a tan suit and took to the stage to Beyonce's Freedom. "We are forever grateful to you."

As he ends his five-decade-long political journey, Biden was surrounded by family, with wife Jill Biden and their daughter Ashley introducing him.

"Joe and I have been together for almost 50 years. And still, there are moments when I fall in love with him all over again," the first lady said.

The Democrats are in an exuberant mood in Chicago, daring to hope for victory in November that seemed inconceivable just weeks ago after Biden's disastrous debate against Trump.

But Monday's first night of the convention saw emotions running high for many of the key players.

Hillary Clinton, who lost against Trump in 2016 in her own bid to become America's first woman president, backed Harris to finally break the glass ceiling.

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"Something is happening in America, you can feel it – something we've worked for and dreamed of for a long time," the former secretary of state and first lady said.

The crowd also chanted "lock him up" as she recalled Trump's recent conviction in a porn star hush money trial – an ironic echo of when Trump led similar chants for Clinton to be jailed in a row over deleted emails.

Harris will formally accept the party's nomination on the final day Thursday, in her biggest chance yet to define her candidacy in front of a huge audience of tens of millions of American voters.

Her star turn will cap a week where the biggest names from the party's past will endorse her as its future, with former president Barack Obama speaking on Tuesday, and ex-president Bill Clinton introducing Harris' running mate Tim Walz on Wednesday.

For Biden, he is suddenly the convention's warm-up act instead of its headliner, and he reportedly still feels frustration at the way he was pushed aside despite feeling that he could still beat Trump.

But he has insisted he is at peace with his decision.

"I am," Biden told reporters earlier when asked if he was ready to pass the torch to Harris.

Protests against Israel's war on Gaza have marked the opening of the convention, underscoring what remains a potential vote-loser for Democrats among progressives, youth, Muslims, and Arab Americans.

A group of demonstrators broke through the outer security fence of the convention after splitting off from a larger protest of thousands of people.

Police in blue helmets with shields and carrying black batons prevented them from getting to the inner cordon.

The mass demonstrations are taking place in response to the Biden-Harris administration's ongoing military support for Israel amid mounting accusations of genocide and war crimes.

Trump, meanwhile, has been sent into a tailspin by the sudden change at the top of the Democratic ticket. While Democrats are in Chicago, the Republican will spend the week crisscrossing the country.

In the battleground state of Pennsylvania on Monday, he highlighted what he called Harris's "craziness" and said she "has no idea what the hell she's doing" on the economy.