An emotional US President Joe Biden passed the torch to Democratic nominee Kamala Harris with a hug on Monday, saying he gave everything for his country in a bittersweet farewell speech at the party's convention in Chicago.

"America, America, I gave my best to you," the 81-year-old Biden said, quoting a patriotic hymn during a nearly hour-long address that ran through his achievements while urging voters to back his vice president against Donald Trump in November.

Harris joined him on stage after the speech and the pair embraced, as the crowd gave Biden a rapturous reception following his stunning decision less than a month ago to drop out of the 2024 White House race.

In a remarkable turnaround, Harris has reenergized the party and wiped out Republican rival Trump's lead in the polls, but Biden insisted that he was not bitter about stepping aside.

Instead, as he contemplates the imminent end of a five-decade political career, he said that he had done what he thought best to ensure that his nemesis Trump does not return to the Oval Office.

"I love the job, but I love my country more. I love my country more," said Biden. "And all this talk about how I'm angry at all those people who said I should step down -- that's not true."

Both Biden and Harris appeared to wipe away a tear as the US leader won a huge four-minute ovation when he first took to the stage, following an introduction by First Lady Jill Biden and his daughter Ashley.

Several members of the audience were also in tears as Biden made his farewell speech, before leaving the stage to the strains of the song "Higher Love."

'Gave my best'

And Harris had earlier made a surprise appearance -- Democratic nominees don't normally speak until the final day of the convention -- to heap lavish tribute on 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."

It was undoubtedly a difficult swan song for Biden, but he insisted he would be the "best volunteer" for Harris's campaign -- knowing perhaps that his legacy depends on her beating Trump.

But he couldn't quite let go of the presidency, with his speech focusing more on his own record in office than the future under a President Harris.

Biden listed his proudest achievements including on the economy and health care, but above all for healing the "soul of America" after Trump's time in office and the pro-Trump January 6, 2021 Capitol riot.

"Donald Trump calls America a failing nation... He says we're losing. He's the loser," he said, also referring to Trump as a "convicted felon" after the Republican was found guilty of doctoring business records to cover up hush money payments to a porn star.

Despite his low popularity ratings and the debate debacle against Trump that led him to step aside, Biden again insisted he'd given his all.

"I made a lot of mistakes in my career, but I gave my best to you," he said.

Protests against Gaza war

As he has been so often in his five-decade-long political journey, Biden was surrounded by family at the end of his speech.

"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 in a speech introducing him.

Monday's first night of the convention was an emotional one on many levels, and 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.

"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.

Earlier, protests against Israel's war in Gaza had shadowed the opening of the convention, underscoring what remains a potential vote-loser for Democrats among left-wingers 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.

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.