US President Donald Trump has turned on his deputy, accusing him of lacking courage after Mike Pence said he would not intervene in the election result.
In a tweet, the president wrote: "Mike Pence didn't have the courage to do what should have been done to protect our Country and our Constitution, giving States a chance to certify a corrected set of facts, not the fraudulent or inaccurate ones which they were asked to previously certify. USA demands the truth!"
Earlier, Mr Trump repeated his claim that the election was stolen from him and called on Mr Pence to "do the right thing" and block the formal confirmation of Joe Biden's presidential win.
— Donald J. Trump (@realDonaldTrump) January 6, 2021
But shortly afterwards, the vice president released a letter confirming he did not have such powers and did not believe those powers should be afforded to anyone in his role.
The president's tweet came as thousands of protesters swarmed around the Capitol building, with some reports of demonstrators breaching security and entering the building.
Washington DC's mayor has since ordered a curfew in the city.
Both the House of Representatives and the Senate have been suspended following the unrest.
Shortly after his criticism of Mr Pence was posted and people stormed the Capitol building, the outgoing president sent a second tweet asking his supporters to support law enforcement.
The post said: "Please support our Capitol Police and Law Enforcement. They are truly on the side of our Country. Stay peaceful!"
Mr Trump sent a further tweet shortly after, repeating his call for peace.
"I am asking for everyone at the U.S. Capitol to remain peaceful. No violence! Remember, WE are the Party of Law & Order - respect the Law and our great men and women in Blue. Thank you!" he wrote.
Congress had been affirming Joe Biden's win in the 2020 US general election. There was opposition from parts of the Republican Party, but it was expected for proceedings to conclude with a confirmation of the result.
In his letter to President Trump, Mike Pence highlighted his oath to God when taking office.
He said: "Four years ago, surrounded by my family, I took an oath to support and defend the Constitution, which ended with the words, 'so help me God.'
"Today I want to assure the American people that I will keep the oath I made to them and I will keep the oath I made to Almighty God.
"When the Joint Session of Congress convenes today, I will do my duty to see to it that we open the certificates of the Electors of the several states, we hear objections raised by Senators and Representative, and we count the votes of the Electoral College for President and Vice President in a manner consistent with our Constitution, laws and history.
"So help me God."
He added: "It is my considered judgement 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."