Donald Trump supporters broke into the US Capitol building and forced a lockdown as they sought to force Congress to undo President Donald Trump’s election loss.
Rioters clashed with police as lawmakers gathered on Wednesday to confirm the election of Democratic President-elect Joe Biden.
"We want Trump," they chanted in the Capitol corridors.
The ongoing special House and Senate sessions have been recessed as a result.
Hundreds of demonstrators, who had gathered to support Trump’s claims of election fraud, clashed with police and pushed through a fence behind the Capitol building.
Police fired tear gas and ordered several buildings to be evacuated. Senators were being evacuated and some lawmakers were sheltering in their offices.
Representative Elaine Luria said on Twitter "I just had to evacuate my office because of a pipe bomb reported outside. Supporters of the President are trying to force their way into the Capitol and I can hear what sounds like multiple gunshots."
Capitol Police told lawmakers in the House chamber to take gas masks from beneath their seats and prepare to put them on.
Some Republicans are trying to overturn the results in some states but lack sufficient support
Vice-President Mike Pence said he would not block the certification of Biden as president-elect, despite demands from Trump.
Earlier on Wednesday, Trump addressed thousands of supporters in Washington DC and encouraged them to occupy the Capitol and said "we will never concede".
He later urged peace on Twitter: "Please support our Capitol Police and Law Enforcement. They are truly on the side of our Country. Stay peaceful!" he said.
It has been weeks since it was confirmed that Biden won the Electoral College 306-232.
Outgoing President Donald Trump alleged widespread fraud in the November 3 election but has failed to produce any evidence to support his claims.
Republicans have been unsuccessful in their attempts to overturn the results in several key states through the courts.
Trump urged Vice President Mike Pence to delay the confirmation by sending Electoral College votes back to the states to be recertified.
“Mike Pence is going to have to come through for us," Trump said, "and if he doesn’t it’s a sad day for our country."
But Pence does not have the authority to do so.
Pence said in a letter he would not block the certification of Biden as president-elect, despite demands from Trump.