President Joe Biden has cemented the Democratic Party's elevation of Kamala Harris to lead the fight for the White House against Republican Donald Trump with a convention speech that praised his vice president as the best hope for preserving American democracy.

Biden took centre stage on opening night of the Democratic National Convention, drawing an extended standing ovation from the party faithful and delivering a farewell speech to the party he has served for half a century – even with five months left in office.

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.

Taking multiple swipes at Trump, Biden promised to be the "best volunteer" Harris and Minnesota Governor Walz, her vice presidential running mate, had ever seen.

Biden's address in Chicago kicked off a four-day event fueled by enthusiasm for Harris and relief that Biden abandoned his own reelection 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 four more years.

"I love the job, but I love my country more," Biden said, drawing chants of "We love Joe."

Joe Biden gets emotional ovation, joined by family, Kamala Harris and Doug Emhoff. – AP

Biden, who was No.2 to the nation's first Black president, Barack Obama, is urging Democrats to unite behind a candidate who, if triumphant on November 5, would become the first woman, who is also Black and South Asian, to be elected US president.

Harris is riding a historic whirlwind into the convention: her campaign has broken fundraising records, packed arenas with supporters, and turned opinion polls in some battleground states in Democrats' favour.

She joined Biden onstage for a hug after he finished his address.

Biden had hoped just a month ago to deliver a high-profile speech later this week to accept the Democratic nomination for another four-year term. Instead, he will travel to California for a vacation.

Biden dropped his reelection bid after his disastrous debate performance against Trump on June 27 prompted longtime allies, major donors and other party supporters to demand he step aside.

Kamala Harris onstage at the Democratic National Convention in Chicago. – Reuters

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.

He contrasted Harris the prosecutor with Trump, a convicted felon, and catalogued what he called the former president's foreign policy failures, accusing him of bowing down to Russian President Vladimir Putin and leaving NATO in tatters.

Biden rapped Trump for calling the United States a failing nation.

"He’s the loser," Biden said.

Unlike most speakers earlier in the program, Biden acknowledged the anger of thousands of pro-Palestinian protesters that gathered outside the convention hall, saying he has been working nonstop to achieve a ceasefire.

"Those protesters out in the street, they have a point," he said. "A lot of innocent people are being killed on both sides."

Earlier in the evening, Harris, 59, made a surprise appearance, drawing cheers from the crowd as she vowed to defeat Trump.

"Let us fight for the ideals we hold dear and let us always remember, when we fight we win!" Harris said to the roar of the crowd. She will formally accept the nomination on Thursday.

Kamala Harris thanks Joe Biden for his 'historic leadership'. – AP

Democrats also cheered their failed 2016 presidential candidate, Hillary Clinton, whose loss to Trump in 2016 dashed hopes of installing the first woman in the White House.

Clinton, who became the first woman to secure a major US party's presidential nomination, drew a standing ovation as she took the stage.

"The story of my life and the history of our country is that progress is possible, but not guaranteed," said Clinton, who also lost her bid for the 2008 nomination to Obama.

She praised Biden for bringing decency, dignity and competency to the White House.

"And now, we are writing a new chapter in America's story," Clinton said. "Kamala has the character, experience and vision to lead us forward."

Clinton saluted Harris while noting her potential to break the “highest, hardest glass ceiling” to become America’s first female president.

“Together, we’ve put a lot of cracks in the highest, hardest glass ceiling,” Clinton said.

“On the other side of that glass ceiling is Kamala Harris taking the oath of office as our 47th president of the United States. When a barrier falls for one of us, it clears the way for all of us.”

Clinton also saluted Biden for stepping aside, saying: “Now we are writing a new chapter in America’s story.”

Hillary Clinton rouses Democrats, salutes Kamala Harris. – AP

Clinton also took several shots at her former nemesis.

"Donald Trump fell asleep at his own trial, and when he woke up, he made his own kind of history, the first person to run for president with 34 felony convictions," she said to laughter.

Highlighting the party’s generational reach, Clinton, 76, followed New York Representative Alexandria Ocasio-Cortez, 34, who endorsed Harris while delivering the first mention of the war in Gaza from the convention stage, addressing an issue that has split the party’s base ever since Hamas’s October 7 attack and Israel’s resulting offensive.

“And she is working tirelessly to secure a ceasefire in Gaza and bringing the hostages home,” Ocasio-Cortez said of Harris, drawing cheers from the crowd.

Earlier, United Auto Workers President Shawn Fain, mimicking Trump-supporting wrestler Hulk Hogan at the Republican convention last month, tore off his jacket to reveal a T-shirt reading "Trump is a scab. Vote Harris" as the audience cheered.

"The American working class is fired up and fed up," Fain said.

While Democrats gathered for their national convention, thousands of protesters assembled at a nearby park to pressure delegates to drop the party’s military support for Israel’s Gaza offensive.

The pro-Palestinian protesters were fewer than the tens of thousands that organisers had predicted, but a splinter group left the main march and breached a security perimeter near the convention centre, drawing riot police to the site.

Joe Biden speaks during the first night of the Democratic National Convention. – Reuters

Several protesters who had managed to get through the fence were detained and handcuffed by the police.

Authorities said the inner security perimeter surrounding convention site was not breached and there was no threat to those attending the convention.

The march happened just as Biden, who has been the target of intense criticism from pro-Palestinian groups, including the marchers, was doing a walk-through of the largely empty United Center. 

The pro-Palestinian protests injected a note of uncertainty into what is otherwise a week of celebration, with some on the party’s left flank angry over the Biden administration’s support for Israel’s actions in Gaza.

“The Democrats are the ones in power,” said Hatem Abudayyeh, a spokesperson for the “March on the DNC” rally that drew thousands of people.

“It’s their war. They’re responsible for it, they’re complicit and they can stop it.”

The protesters appeared unlikely to pressure Democrats to change.

The party was due to vote on Monday on a 92-page policy platform that does not call for an arms embargo against Israel, a demand of pro-Palestinian groups.

The United States approved $20 billion in additional arms sales to Israel on Tuesday.

Attendees wave USA signs at the Democratic National Convention in Chicago. – Reuters

Harris is riding a historic whirlwind into the convention – her campaign has broken records for fundraising, packed arenas with supporters, and turned opinion polls in some battleground states in Democrats’ favour.

Harris’ vice presidential running mate, popular Minnesota Governor Tim Walz, was greeted with chants of “We’re not going back” when he met with groups of delegates.

One prominent backer, however, cautioned fellow Democrats not to get too optimistic.

“Our numbers are much less rosy than what you’re seeing in public,” said Chauncey McLean, who heads Future Forward, a committee that has raised hundreds of millions of dollars to help elect Harris.

Protesters break through police fence near site of Democratic National Convention. – AP

Trump, meanwhile, plans to campaign this week in battleground states that are likely to determine the outcome of the election.

Some major allies and donors have been urging Trump to steer clear of racial and gender-based insults of Harris and focus his attacks instead on her policy record.

At a small business in southern Pennsylvania, he repeatedly referred to Harris as “Comrade Kamala” in an effort to paint her as a communist at an event to discuss economic policies.

On social media, he said he would reduce crime in cities like Chicago. “THIS IS A WAR ZONE, AND WILL BE HANDLED ACCORDINGLY,” he wrote on social media.

Democrats hope to extend Kamala Harris's presidential momentum at DNC. – AP