Kamala Harris warned against Donald Trump's bid to seize "unchecked power" in a fiery speech on 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 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.

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 the 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 crowd extended far beyond the Ellipse, across the Mall and all the way up to the iconic Washington Monument obelisk.

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.

"I think of this as a cleansing for what happened on January 6," said Mitzi Maxwell, 69, who came from Florida with her mother to see "all the love and passion and excitement that she (Harris) has become known for."

Some Harris supporters queued 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.

As Harris spoke in Washington, Trump visited a heavily Hispanic city in Pennsylvania, two days after comedian Tony Hinchcliffe's comments about Puerto Rico drew outrage at the New York rally.

Puerto Ricans are the largest Hispanic group in Pennsylvania, the most crucial battleground state to win as it holds the highest number of Electoral College votes of the seven battleground states, according to the Census Bureau.

"I’d like to begin with a very, very simple question: Are you better off now than you were four years ago? I’m here today with a message of hope for all Americans," Trump said. (Agencies)