President-elect Joe Biden has called for the restoration of "simple decency" as a mob incited by his predecessor stormed the US Capitol and delayed Congress from certifying the results of November's election.
Biden had planned to deliver a speech focused on how to revive the economy and provide financial relief for small-business owners reeling from the coronavirus pandemic, giving routine political remarks from a theatre in his native Delaware.
But shortly before he was to begin speaking on Wednesday, demonstrators broke into the Capitol building, reaching as far as the Senate floor.
"Our democracy is under unprecedented assault unlike anything we've seen in modern times," Biden said adding that the violent and chaotic events were "an assault on the rule of law".
The Capitol building was locked down and police moved in with guns drawn as Vice President Mike Pence and lawmakers were evacuated to secure locations.
National Guard troops were deployed and a citywide curfew called for shortly after dusk, as rioters continued to occupy the seat of Congress for hours.
"I call on this mob to pull back and allow democracy to go forward," said the president-elect.
In an address that took only about 10 minutes and was televised against a split screen of the still-occupied Capitol building, Biden attempted to project calm and to say that a deeply divided country can still come together.
He returned to themes that were a centrepiece of his presidential campaign, including finding common political ground, and pledged to be president for all Americans, even those who didn't vote for him.
But Biden also expressed shock and outrage.
"This is not decent, it's chaos," he said.
The unrest erupted as a joint session of Congress had convened to certify Biden's election victory over President Donald Trump.
Biden stopped short of accusing Trump of treason but said what happened "borders on sedition".
"I call on this mob to pull back and allow the work of democracy to go forward," the president-elect said.
"The words of a president matter, no matter how good or bad that president is," Biden said. "At their best, the words of a president can inspire. At their worst, they can incite."
Biden called on Trump to "go on national television now, to fulfil his oath and defend the Constitution and demand an end to this siege".
Biden has said previously that pursuing criminal action against Trump might not be "good" for American democracy, though he wouldn't block future, independent Justice Department investigations into Trump once he leaves office.
There may be calls now for Biden to rethink that.
Biden's original speech was delayed by more than an hour as his aides tore up the original, scheduled remarks and worked to craft a new statement that could address what was unfolding in the nation's capital.
There didn't appear to be additional security around Biden or his motorcade. But as the president-elect huddled backstage, agents on his US Secret Service detail stood in their places in front of the stage where he was set to speak, which featured four American flags, for more than hour.
Upon beginning his speech, Biden said the events at the Capitol "do not reflect a true America. Do not represent who we are."
"The work of the moment and the work of the next four years must be the restoration of democracy, of decency, of honour, of respect, the rule of law," he said.
"Just plain, simple decency. The renewal of a politics that's about solving problems, looking out for one another, not stoking the flames of hate and chaos."
Australian Associated Press