Chicago, Illinois - Kamala Harris accepted the Democratic presidential nomination in Chicago on Thursday before a rapturous crowd, pledging a "new way forward" and warning that Donald Trump will take the US backward if he wins in November.

"With this election, our nation has a precious, fleeting opportunity to move past the bitterness, cynicism, and divisive battles of the past – a chance to chart a new way forward," Harris said to huge cheers from tens of thousands of pumped-up supporters.

"And I want you to know: I promise to be a president for all Americans."

The convention became a giant party to celebrate Harris's astonishing ascent from something of a political afterthought to Democratic standard-bearer upon President Joe Biden's decision to end his faltering reelection bid.

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A sea of waving stars and stripes flags and chants of "USA" filled the arena as jubilant Democrats anointed Harris.

She was later joined on stage by her running mate Tim Walz and their families, as they held their arms aloft while 100,000 red, white and blue balloons tumbled from the ceiling.

The Chicks sang a version of The Star-Spangled Banner, while pop star Pink also performed as the Democrats rolled out a list of celebrity backers.

It was Harris's time to shine on the biggest night of her political life, after becoming the first Black woman to be nominated by a major US party.

She promised to bring economic opportunity and protect their personal freedoms on key issues like abortion.

"I will be a president who unites us around our highest aspirations," she vowed.

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Harris then launched a broadside at Trump, whose campaign has been upended by having to face a woman two decades younger, rather than the increasingly frail Biden, 81.

"We know what a second Trump term would look like," she said, saying he wanted to "pull our country back to the past."

She laid out her personal story as a child of a single working mother, and her career as a prosecutor, saying she has the background and experience to serve the country, in contrast to Trump, who she said only works for himself and "his billionaire friends."

Turning to foreign policy, she accused Trump of trying to "cozy up" to foreign autocrats like Russian President Vladimir Putin.

Harris pledged instead to "stand strong" with Ukraine as it fights Russia's invasion and support NATO allies – all in stark contrast to Trump's stance.

On the issue of Israel's brutal, US-aided war on Gaza, Harris called the scale of suffering in the Palestinian enclave "heartbreaking" – but only after pledging to always stand by Israel's side.

She vowed to get a ceasefire deal done, and cheers erupted when she advocated for "self-determination" for the Palestinian people.

Protesters have demonstrated in solidarity with Gaza throughout the Democratic convention, with several thousands rallying outside it again on Thursday.

Yet the DNC denied Palestinians a voice at the convention, despite featuring speakers from every imaginable constituency – including the family of an Israeli soldier held prisoner in Gaza.

The Harris campaign has made it clear she opposes an arms embargo, despite US-made bombs being used in documented case after documented case of alleged Israeli war crimes and crimes against humanity.