Donald Trump's supporters on Wednesday stormed a session of Congress held to certify Joe Biden's win, as a desperate last-minute bid by the president to overturn his election loss sparked chaos and accusations of a "coup" attempt.

Hours after an extraordinary rally by Trump challenging his defeat, his flag-waving backers broke down barricades outside the Capitol and swarmed inside, with the special session going into an emergency recess as protesters entered the chambers.

Trump supporters, waving his blue flags and wearing his red campaign hats, stormed through the building, making it right into the debating chamber. A viral photo on Twitter showed plainclothes security men aiming pistols point-blank through the smashed window of a door to stop anyone else getting in.

Legislators were given gas masks to protect themselves against tear gas as they rushed to safety.

For those fleeing, it was a race against time: Protestors were getting in as quickly as members of Congress could get out. Some protestors even occupied the office of House Speaker Nancy Pelosi, sitting mockingly at a desk.

One person was shot, Washington D.C's police chief said. The city's mayor ordered a 12-hour curfew starting at 6pm (12am Malta time). 

White House spokeswoman Kayleigh McEnany said in a tweet that Trump had directed the callup of National Guard troops "along with other federal protective services" to deal with the unrest.

"We reiterate President Trump's call against violence and to remain peaceful," she said.

Biden denounces 'insurrection'

President-elect Biden denounced the storming of the US Capitol as an "insurrection" and demanded that President Trump go on television to call an end to the violent "siege."

"At this hour, our democracy is under unprecedented assault," Biden said. 

"I call on President Trump to go on national television now to... demand an end to this siege," Biden said. "To storm the Capitol, to smash windows, to occupy offices, the floor of the United States Senate... threatening the safety of duly elected officials? It's not a protest, it's insurrection."

Trump urges people to 'go home'

Trump released a one-minute video statement on Twitter in which he insisted that the election was "fraudulent" but urged people to go home.  

"We can't play into the hands of these people," he said. 

"I know your pain," Trump said. "We had an election that was stolen from us.

"But you have to go home now," he said. "We have to have peace. We have to have law and order."

Governments across Europe and the European Union's foreign policy chief condemned the protesters.

"Disgraceful scenes in U.S. Congress. The United States stands for democracy around the world and it is now vital that there should be a peaceful and orderly transfer of power," UK Prime Minister Boris Johnson said on Twitter.

The chaos at the Capitol, which was put under lockdown, came one day after Biden enjoyed a new triumph as his Democratic Party won two Senate runoff seats, handing full control of Congress. 

Police fired tear gas as they struggled to remove a sea of protesters who took aim, at Trump's urging, at what for more than two centuries had been a quiet, ceremonial session to confirm the election's winner.

As protestors stormed the building and crowded around it, President Donald Trump ordered the National Guard to the Capitol, his press secretary said on Twitter.

Democrat politicians described the scenes as a "coup". 

"The President of the United States is inciting a coup. We will not be intimidated. We will not be deterred," tweeted Democratic Representative Karen Bass.

Representative Val Demings likewise denounced the storming of the Capitol as evidence of "a coup in progress" -- in words echoed by half a dozen lawmakers.

Representative Elaine Luria said she had to evacuate because of a report of a pipe bomb and said she believed she heard gunshots.

"I don't recognize our country today and the members of Congress who have supported this anarchy do not deserve to represent their fellow Americans," she said.

Damage our republic forever

Biden is certain to become president, with Democrats already controlling the House of Representatives, but more than 140 Republican House members and a dozen Republican senators have sided with Trump in challenging the results even though no evidence of fraud has been proven in court.

Republican Senate leader Senator Mitch McConnell, for four years a staunch backer of Trump, denounced the challenge in an impassioned address, noting that the results of the November 3 vote were not even close.

"The voters, the courts and the states have all spoken. If we overrule them, it will damage our republic forever," said McConnell, who is set to lose his position of majority leader after Democrats' Senate wins.

"If this election were overturned by mere allegations from the losing side, our democracy would enter a death spiral," said McConnell, who also accused Democrats of not accepting Trump's surprise 2016 victory.

Senator Chuck Schumer voiced alarm moments before protesters entered the Capitol, saying: "An element of the Republican Party believes their political viability hinges on the endorsement of an attempted coup."

'We will never concede' 

As scenes of chaos spread, Trump took to Twitter to call on his supporters to be peaceful and Washington Mayor Muriel Bowser imposed a 6 pm curfew in the city. 

But in his rally near the White House, Trump urged them to head to the Capitol.

Rambling angrily, the president warned "weak" Republicans not to certify Biden's victory and put direct pressure on Vice President Mike Pence, who ceremonially presided over the session.

"We will never give up. We will never concede," Trump told the cheering crowd, few wearing masks despite a spike in Covid cases.

"I hope Mike has the courage to do what he has to do," Trump said, describing the US election as less honest than those of "Third World countries."

As Trump was still speaking and Congress opened the session, Pence - dutifully loyal to Trump over four years and quiet since the election - said he would not intervene.

"The Constitution constrains me from claiming unilateral authority to determine which electoral votes should be counted and which should not," Pence said in a statement.

Thousands of Trump supporters had headed to Washington at his urging in recent days, with downtown businesses boarding up in fear of violence.

"I can't say I respect our election process anymore," said Gail Shaw, 76, who drove down from New Jersey. "We will take our nation back."

Biden won more than seven million votes more than Trump in the November 3 election and leads him 306-232 in the state-by-state Electoral College that determines elections.

Trump has repeatedly alleged without no evidence that there was vote-rigging but his team has not been able to prove a single case in court.

New mandate for Democrats

The session of Congress comes one day after voters went to the polls in Georgia handed a pair of stunning victories to the Democratic Senate candidates over Republican incumbents.

The two seats guarantee the Democratic party control of the body, with the 50-50 split in the Senate tipping their way thanks to the tie-breaking vote of Vice President-elect Kamala Harris.

Biden is due to be sworn in on January 20 and control of the Senate gives his Democrats the levers of power in the executive branch and both chambers of Congress, allowing him to push through his legislative agenda.

"After the past four years, after the election, and after today's election certification proceedings on the Hill, it's time to turn the page," Biden said in a statement.

"The American people demand action and they want unity. I am more optimistic than I ever have been that we can deliver both," he said.

In Georgia, Democratic candidate Raphael Warnock, the pastor at the Atlanta church where Martin Luther King once preached, defeated Republican Kelly Loeffler, a 50-year-old businesswoman appointed to the Senate in December 2019.

Warnock, 51, will be just the third African-American to win a Senate seat from the South.

In the other Georgia race, Democrat Jon Ossoff, a 33-year-old video producer, claimed victory on Wednesday over Republican David Perdue.