Boris Johnson has said it was "completely wrong" of Donald Trump to "encourage people to storm the Capitol" - as figures from across the political spectrum criticised the US president's actions on Wednesday.
The UK prime minister was responding to a questions at a Downing Street coronavirus briefing on Thursday when he made the comments.
He said: "All my life America has stood for some very important things - an idea of freedom and an idea of democracy.
"As you suggest, in so far as he [President Trump] encouraged people to storm the Capitol and in so far as the president has consistently cast doubt on the outcome of a free and fair election, I believe that that was completely wrong.
"What President Trump has been saying about that is completely wrong and I unreservedly condemn encouraging people to behave in the disgraceful way that they did in the Capitol.
"All I can say is that I'm very pleased that the president-elect has now been duly confirmed in office and that democracy has prevailed."
His comments came after former US president Barack Obama accused Mr Trump of inciting his supporters who stormed the US Capitol - describing it as a "moment of shame".
In a statement on Twitter, the Democrat said: "History will rightly remember today's violence at the Capitol, incited by a sitting president who has continued to baselessly lie about the outcome of a lawful election, as a moment of great dishonor and shame for our nation.
"For two months now, a political party and its accompanying media ecosystem has too often been unwilling to tell their followers the truth - that this was not a particularly close election and that president-elect Biden will be inaugurated on January 20.
"Their fantasy narrative has spiralled further and further from reality, and it builds upon years of sown resentments.
"Now we're seeing the consequences, whipped up into a violent crescendo."
Former Republican president George W Bush also condemned the actions of Mr Trump and his supporters, branding the unrest "mayhem" and the acts of a "banana republic".
President Bush, who led the US from 2001 to 2009, did not mention the current commander-in-chief by name in his statement.
"This is how election results are disputed in a banana republic - not our democratic republic," he said.
"I am appalled by the reckless behaviour of some political leaders since the election and by the lack of respect shown today for our institutions, our traditions, and our law enforcement.
"The violent assault on the Capitol - and disruption of a Constitutionally-mandated meeting of Congress - was undertaken by people whose passions have been inflamed by falsehoods and false hopes.
"Insurrection could do grave damage to our nation and reputation."
People who have worked inside the Trump administration were also excoriating of the man currently trying to cling on to power.
Former chief of staff Mick Mulvaney said that the president's messages on Twitter were insufficient.
He said: "The President's tweet is not enough. He can stop this now and needs to do exactly that. Tell these folks to go home."
Elsewhere, there were calls for the 25th amendment of the US Constitution to be invoked to remove Mr Trump from power as soon as possible.
The Republican governor of Vermont, Phil Scott, said: "The fabric of our democracy and the principles of our republic are under attack by the President.
"Enough is enough.
"President Trump should resign or be removed from office by his Cabinet, or by the Congress."
The fourth article of the 25th amendment allows the vice president and the US cabinet to vote on the removal of the president.
President Trump could dispute the action, but the issue could still be forced through should other members of the executive again vote to remove him.
There would then need to be votes in both the House of Representatives and the Senate, with the order to remove requiring a two-thirds majority.
Other people have called for an impeachment, like what the president faced a year ago.
Democratic House member Ilhan Omar said: "I am drawing up Articles of Impeachment.
"Donald J. Trump should be impeached by the House of Representatives & removed from office by the United States Senate.
"We can't allow him to remain in office, it's a matter of preserving our Republic and we need to fulfil our oath."
There was further condemnation from around the world from the likes of Boris Johnson, president of the European Commission Ursula von der Leyen, Joe Biden and Canadian Prime Minister Justin Trudeau.
Earlier in the day, Vice President Mike Pence said he did not have the constitutional power to overrule the electoral college, drawing the ire of his superior in the Oval Office.
And Senate leader Mitch McConnell - another member of the Republican Party turning their back on President Trump - told his colleagues that the election was over and "was not even particularly close".