Washington DC - Kamala Harris warned against Donald Trump's bid to seize "unchecked power" in a fiery speech Tuesday at the spot where her rival riled up a mob before the deadly January 6, 2021 assault on the US Capitol.

"This is someone who is unstable, obsessed with revenge, consumed with grievance, and out for unchecked power," she said.

"But America, I am here tonight to say: that's not who we are," Harris told a huge crowd of flag-waving supporters against the imposing backdrop of the White House in Washington.

The campaign claimed that 75,000 people attended the rally, which comes just a week before the current Democratic vice president faces the Republican former president in the closest and most volatile election of modern times.

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The number could not be immediately verified, but the crowd was unusually big in an election that has already seen heavy enthusiasm.

The choice of the Ellipse – a park linking the White House to the vast National Mall – was a direct attempt by her campaign to remind voters of the chaos caused by Trump's attempt to overturn his 2020 election loss to Joe Biden.

After Trump urged supporters in a speech there to "fight like hell," many then marched on the Capitol to disrupt the certification of Biden's victory, in an assault that left 140 police officers wounded and shocked the world.

The choice of venue – the White House lit up against the night in the background – was also a symbolic pitch to show Harris ready for the presidency.

Speaking from behind bulletproof screens next to blue signs saying "Freedom," Harris pledged to be a "president for all Americans" – unlike Trump, whom she accused of wanting to jail his enemies.

And although her speech began with a dramatic put-down of Trump's behavior, she soon pivoted to a recap of her detailed plans to help financially struggling middle-class Americans.

She got one of the biggest cheers when she referred to Republicans' drive to curtail abortion, saying the government should not be "telling women what to do with their bodies."

The pumped-up crowd extended far beyond the Ellipse, across the Mall, and all the way up to the iconic Washington Monument obelisk, AFP reporters said.

Although there is almost a week to go, the Harris campaign cast the speech as a "closing argument" – a nod to her career as a prosecutor.

Some Harris supporters stood in line for more than seven hours before the speech, whose sheer scale and energy was a direct challenge to Trump, a politician who has always boasted about his ability to draw crowds.

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Trump tried to take the wind out of Harris's speech with a campaign event of his own, in front of a room of supporters at his Mar-a-Lago resort in Florida.

His remarks came as he was seeking to tamp down the firestorm over his weekend rally in New York's famed Madison Square Garden, where a warm-up comedian jibed that Puerto Rico was a "floating island of garbage."

Trump called the New York event a "love fest," the same language he has used to describe the Capitol riot.

He also lashed out at Harris.

"Her message has been a message of hate and division," said Trump.

The Republican later rallied in blue-collar Allentown, in Pennsylvania, perhaps the most crucial of the seven battleground states that are expected to decide the election – and a city that is home to a large Puerto Rican community.

Fears of a repeat of the chaos from four years ago hang heavy over this year's election, with Trump repeatedly indicating that he might again refuse to accept the result if he loses.

On Tuesday, Trump seized on reports of Pennsylvania authorities halting hundreds of potentially fraudulent voter registration forms.

"Really bad 'stuff.' WHAT IS GOING ON IN PENNSYLVANIA???" he wrote on X.

More than 50 million people have already cast their ballots early, figures showed on Tuesday – already nearly a third of the total number of voters four years ago.

The last days of the campaign will see both candidates on a grueling final swing through the battlegrounds.

In a bid to reach out to young and Latino voters, Harris' campaign announced that actor and singer Jennifer Lopez is to join Harris in Las Vegas, Nevada on Thursday.