Angry supporters of President Donald Trump have stormed the US Capitol in a chaotic protest aimed at thwarting a peaceful transfer of power.
The attack forced politicians to rush from the building and interrupted challenges to Joe Biden's Electoral College victory.
President Trump issued a restrained call for peace well after the protests was under way but did not urge supporters to disperse. Earlier he had seemingly egged them on to march to Capitol Hill.
In a raucous, out-of-control scene, protesters fought past police and breached the building, shouting and waving Trump and American flags as they marched through the halls.
Police said four people died in the protests. Washington DC police chief Robert Contee said the dead included a woman who was shot by US Capitol Police, as well as three others who died in "medical emergencies".
The woman was shot as the mob tried to break through a barricaded door in the Capitol where police were armed on the other side. She was hospitalised with a gunshot wound and later died.
Police said both law enforcement and protesters deployed chemical irritants during the hours-long occupation of the Capitol building before it was cleared by law enforcement.
DC police officials also said two pipe bombs were recovered, one outside the Democratic National Committee and one outside the Republican National Committee.
The protesters abruptly interrupted the congressional proceedings in an eerie scene that featured official warnings directing people to duck under their seats for cover and put on gas masks after tear gas was used in the Capitol Rotunda.
With the crowds showing no signs of abating, President Trump tweeted: "Please support our Capitol Police and Law Enforcement. They are truly on the side of our Country. Stay peaceful!"
Earlier, at his rally, he had urged supporters to march to the Capitol.
Senators were being evacuated. Some House politicians tweeted they were sheltering in place in their offices.
Vice President Mike Pence was closely watched as he stepped onto the dais to preside over the joint session in the House chamber.
Mr Pence has a largely ceremonial role, opening the sealed envelopes from the states after they are carried in mahogany boxes used for the occasion, and reading the results aloud.
But he was under growing pressure from Trump to overturn the will of the voters and tip the results in the president's favour, despite having no legal power to affect the outcome.
"Do it Mike, this is a time for extreme courage!" Mr Trump tweeted.
But Mr Pence, in a statement shortly before presiding, defied Mr Trump, saying he could not claim "unilateral authority" to reject the electoral votes that make Mr Biden president.
As darkness began to set in, law enforcement officials worked their way toward the protesters, using percussion grenades to try to clear the area around the Capitol.
Big clouds of tear gas were visible. Police in full riot gear moved down the steps, clashing with demonstrators.
The Pentagon said about 1,100 District of Columbia National Guard members were being mobilised to help support law enforcement at the Capitol.