WASHINGTON - Hours after a pro-Trump mob stormed the US Capitol, members of the House of Representatives and Senate resumed a politically charged debate over the legitimacy of the presidential election that had helped stoke the violence.

House Speaker Nancy Pelosi had said in a letter to members that the House would resume as early as Wednesday night (Jan 6) once the Capitol was declared safe.

"We always knew this responsibility would take us into the night," Ms Pelosi said in the letter. "The night may still be long but we are hopeful for a shorter agenda, but our purpose will be accomplished."

Senate Majority Leader Mitch McConnell urged Senators to return to their work in a demonstration of strength in reaction to the demonstrations which were encouraged by President Donald Trump, a person familiar with the matter said.

The United States Capitol went into lockdown hours earlier as pro-Trump protesters stormed barricades and breached the historic building, forcing Congress to suspend an ongoing debate on Republicans’ attempts to overturn the electoral victory of US President-elect Joe Biden.

"In this hour, our democracy is under unprecedented assault, unlike anything we’ve seen in modern times. An assault on the citadel of liberty, the Capitol itself," said Mr Biden in a brief media appearance from Wilmington, Delaware.

He added: "This is not dissent. It's disorder, it's chaos. It borders on sedition, and it must end now. I call on this mob to pull back and allow the work of democracy to go forward."

Smashing windows to enter the building, throngs of right-wing protesters fought with Capitol police once inside, brandishing Trump banners and confederacy flags and cheering from the balconies of the Capitol.

Members of Congress were told to put on the gas masks under their seats, after tear gas was deployed in the Capitol Rotunda.

Multiple officers were reportedly injured and one woman was said to have died after being shot inside the Capitol, according to media reports citing the DC Metropolitan Police Department.

Metropolitan Police Department Chief Robert Contee said three others died on the US Capitol grounds because of medical emergencies.

Police have arrested 52 people, with several of them on charges related to carrying unlicensed or prohibited firearms. Two pipe bombs were also recovered from the headquarters of the Republican and Democratic national committees, as well as a cooler box from a vehicle on US Capitol grounds that contained Molotov cocktails, Reuters reported.

Wednesday’s extraordinary scenes were the culmination of months of conspiracy theories. They were fuelled by President Donald Trump’s refusal to concede the election even after losing the popular vote and the electoral college vote.

Mr Biden said that the words of a President mattered, and at their worst could incite, urging Mr Trump to step up and demand an end to the "siege".

Just hours before, Mr Trump appeared at a rally in front of thousands of protesters, again claiming that the election had been stolen from him.

He urged them to march on the Capitol to "cheer on" Congress and "show strength", promising he would be there alongside them. He was not, and in a tweet issued after protesters forced their way into the Capitol, Mr Trump asked "everyone at the US Capitol to remain peaceful. No violence!"

After being urged to call for an end to the violence, Mr Trump released a brief video on Twitter repeating his false claims of electoral fraud. "We had an election that was stolen from us...but you have to go home now. We have to have peace. We have to have law and order," he said.

"There’s never been a time like this, where such a thing happened - where they can take it away from all of us," Mr Trump added. "This was a fraudulent election, but we can’t play into the hands of these people."

Twitter later suspended Mr Trump from tweeting for 12 hours and threatened to permanently ban him from the platform for flouting its civic integrity rules.

The violence on display has raised questions about America’s peaceful transition of power, a once-inconceivable turn of events in a superpower that has styled itself a champion of democracy elsewhere.

"This is what the president has caused today, this insurrection," Republican senator Mitt Romney of Utah, a frequent Trump critic, told the New York Times shortly after he and other senators were evacuated.

"This violence was the inevitable and ugly outcome of the President's addiction to constantly stoking division," said Republican senator Ben Sasse of Nebraska. "This is not how we peacefully transfer power."

Other lawmakers, both Republicans and Democrats, took to Twitter to label the violence a "coup attempt".

Before the evacuation, Congress had been debating the first objection to the ceremonial count of electoral college votes, raised by a group of Republican senators and Congressmen.

Vice-President Mike Pence, who presided over the proceedings in a ceremonial role as President of the Senate, had been publicly pressured by Mr Trump to overturn states’ votes.

But Mr Pence acknowledged that the Constitution does not give him the power to do so, in a letter sent to Congress and released ahead of the joint session.

"It is my considered judgment that my oath to support and defend the Constitution constrains me from claiming unilateral authority to determine which electoral votes should be counted and which should not," said Mr Pence.

For that, Mr Trump criticised him later on Twitter, saying that he "didn’t have the courage to do what should have been done".

Senate majority leader Mitch McConnell rebuked his colleagues in a speech that explained why he would not vote in favour of the option, saying: "The voters, the courts and the states have all spoken... If we overrule them all, it would damage our republic forever."

"If this election were overturned by mere allegations from the losing side, our democracy would enter a death spiral," he added.

Though the objections do not have the number to succeed, they have exposed a rift in the Republican Party. Some Republicans slammed their colleagues - and Mr Trump - for giving people false hope that Congress could overturn the defeat on Jan 6, and indirectly encouraging the violence on Wednesday.

"The objectors need to stop meddling with the primal forces of our democracy here. There is a cost. They think they’re just having a protest debate and they can get away with it because it’s not actually going to overturn the election. Now we’re seeing the cost of that play out in real time," said Republican congressman Mike Gallagher of Wisconsin, who urged the President to "call it off".

Democratic leaders Nancy Pelosi and Chuck Schumer also issued a joint statement, calling on President Trump to demand that all protesters leave the US Capitol and Capitol Grounds immediately.

Republican former president George W. Bush weighed in in a statement, calling the "scenes of mayhem" an insurrection and "how election results are disputed in a banana republic". "The violent assault on the Capitol...was undertaken by people whose passions have been inflamed by falsehoods and false hopes," Mr Bush wrote.

Washington DC’s mayor announced a 6pm citywide curfew and the DC National Guard was deployed.

Protesters were gradually dispersed from the Capitol grounds and the complex was declared secure by officials just before 6pm, about four hours after they first broke in. However, hundreds of Trump supporters were still on the streets of DC, after the curfew went into effect.