Capitol riots: Boris Johnson calls Donald Trump 'completely wrong'
UK prime minister condemns departing US president for inciting violence
"All my life, America has stood for some very important things: an idea of freedom and an idea of democracy," said Mr Johnson, who was born in New York.
"Insofar as he encouraged people to storm the Capitol and insofar as the president consistently has cast doubt on the outcome of a free and fair election, I believe that was completely wrong.
"And I unreservedly condemn encouraging people to behave in the disgraceful way that they did in the Capitol.
"And all I can say is I'm very pleased that the president-elect [Joe Biden] has now been properly confirmed and duly confirmed in office and that democracy has prevailed."
Mr Johnson was the first European leader to speak to Mr Biden after his November election victory.
The president-elect had previously described the UK prime minister as a "physical and emotional clone" of Donald Trump.
But with a UK-US trade deal at stake, Mr Johnson will be keen to build bridges, not walls.
On Thursday, Mr Biden called the riots “one of the darkest days in the history of our nation".
Former Democratic president Jimmy Carter released a statement through the Carter Centre calling the riots a "national tragedy".
And Bill Clinton, who held the office from 1993 to 2001, called the violence "an unprecedented assault on our Capitol, our Constitution and our country".
World leaders round on Trump
World leaders also did not hold back in censuring Mr Trump.
"This is not America," said French President Emmanuel Macron.
German Chancellor Angela Merkel said: "These images made me angry and sad."
New Zealand Prime Minister Jacinda Ardern decried the attack on democracy.
"What is happening is wrong," Ms Ardern said. "Democracy – the right of people to exercise a vote, have their voice heard and then have that decision upheld peacefully – should never be undone by a mob."
This video rounds up the withering responses of global leaders to the riots.
Washington riots: rogues gallery
Updated: January 8, 2021 12:48 AM