Hundreds of supporters of President Donald Trump stormed the US Capitol in a bid to overturn his election defeat, forcing Congress to postpone a session that would have certified President-elect Joe Biden’s election victory.

A woman has died after being shot during the storming, a spokeswoman for the Washington police department said.

According to the American Broadcasting Corporation, the woman was a rally-goer who pulled out her phone to film and was shot by Capitol police.

Police officers in riot gear line up as protesters gather on the US Capitol Building. Photo: Getty

The chaos unfolded amid a last-ditch effort by some of Mr Trump’s fellow Republicans to throw out the results.

Police evacuated the House of Representatives and the Senate on Wednesday (local time) after pro-Trump protesters marched through the halls of Congress.

One protester occupied the Senate dais and yelled, “Trump won that election.”

Mr Biden said the riots by supporters of Mr Trump were not a protest, characterising them as an insurrection.

In an address to the country, he called on Mr Trump to “fulfil his oath, to defend the Constitution, and demand an end of this siege,”

“Our democracy is under unprecedented assault unlike anything we’ve seen in modern times,” he said.

“Assault on the citadel of liberty, the Capitol itself. An assault on the people’s representatives, on the Capitol Hill police sworn to protect them, on the public servants who work at the heart of our republic.

“An assault on the rule of law, like few times we’ve ever seen it. An assault on the most sacred of American undertakings, the doing of the people’s business.”

Prime Minister Scott Morrison joined world leaders in condemning the rioters, taking to social media to express his disapproval and support for the results of the 2020 election.

Tweet from @ScottMorrisonMP

Tweet from @BorisJohnson

Video showed protesters breaking windows and police deploying tear gas inside the building.

The chaotic scenes unfolded after Mr Trump, due to leave office on January 20, addressed thousands of protesters, repeating false claims that the election was stolen from him due to widespread election fraud and irregularities.

Lawmakers had been debating a last-ditch effort by pro-Trump lawmakers to challenge the results, an effort that was unlikely to succeed.

Vice-President Mike Pence, who had presided over the joint session of Congress, had already been escorted from the Senate.

Capitol Police told lawmakers in the House chamber to take gas masks from beneath their seats and prepare to put them on. Officers at the front door of the House chamber had their guns drawn as someone attempted to enter the chamber.

Officers ordered people in the chamber to drop to the floor for their safety.

Twitter has suspended Mr Trump’s account, threatening to lock the President out if tweets it has deemed in violation of its terms are not removed.

Tweet from @TwitterSafety

Facebook and YouTube took down a video from Mr Trump that continued to make the baseless claim the election was fraudulent.

Facebook’s vice-president of integrity Guy Rosen on Wednesday tweeted the social media company believed the video “contributes to rather than diminishes the risk of ongoing violence”, saying the action was part of “appropriate emergency measures”.

Google-owned YouTube said the video violated its policy against content that alleges “widespread fraud or errors changed the outcome of the 2020 U.S. Election”. YouTube spokesman Farshad Shadloo added the company does allow copies that include additional context.

Twitter restricted users from retweeting the video “due to a risk of violence”.

A group of pro-Trump protesters raise an America flag on the West grounds of the Capitol Building. Photo: Getty

Election officials of both parties and independent observers have said there was no significant fraud in the November 3 contest, which Mr Biden won by more than seven million votes in the national popular vote.

Weeks have passed since the states completed certifying that Mr Biden, a Democrat, won the election by 306 Electoral College votes to Mr Trump’s 232. Mr Trump’s extraordinary challenges to Mr Biden’s victory have been rejected by courts across the country.

Mr Trump had pressed Mr Pence to throw out election results in states the president narrowly lost, though Mr Pence has no authority to do so.

The certification in Congress, normally a formality, was expected to stretch for several hours as some Republican lawmakers mounted an effort to reject some state tallies, starting with Arizona.

Senate Majority Leader Mitch McConnell denounced the effort, saying, “If this election were overturned by mere allegations from the losing side, our democracy would enter a death spiral.”

Mr McConnell helped give Mr Trump some of the biggest accomplishments of his presidency, including deep tax cuts and confirmation of conservative judicial nominees.

The voters, courts and states “have all spoken”, Mr McConnell said on the Senate floor. “If we overrule them, it would damage our republic forever,” he added.

Senate Democratic Leader Chuck Schumer called the challenges “an attempted coup” and said “the Congress does not determine the outcome of an election. The people do.”

Riot police prepare to move demonstrators away. Photo: AFP

Outside the Capitol, members of militia groups and far-right groups, some in body armour, mingled with the crowds. Protesters chanting pro-Trump slogans overturned barricades and clashed with police.

On Twitter, Mr Trump had called on protesters to stay peaceful.

Tweet from @realDonaldTrump

Washington Mayor Muriel Bowers ordered a citywide curfew starting at 6pm.

Mr Pence has rebuffed Mr Trump’s demand that the Vice-President unilaterally reject state electoral votes on the same day Mr Trump’s fellow Republicans were poised to lose their majority in the Senate.

“We will never give up,” Mr Trump earlier told thousands of cheering supporters on a grassy expanse near the White House called the Ellipse. “We will never concede. It doesn’t happen. You don’t concede when there’s theft involved.”

Mr Trump eventually issued a video address via Twitter, telling the rioters “it’s time to go home. We need peace.”

pic.twitter.com/Pm2PKV0Fp3

— Donald J. Trump (@realDonaldTrump) January 6, 2021

Mr Trump had earlier applied fresh pressure on Mr Pence to try to reverse the election results.

In a statement, Mr Pence said he shares the concerns about the “integrity” of the election but that is not correct that he should be able to accept or reject electoral votes unilaterally.

The US Constitution does not give Pence the power to unilaterally overturn the results of the election, but he is under pressure to do so from Mr Trump.

-with AAP