Four people were stabbed and one shot in the stomach as protests against the election result descended into violence on Saturday night.
Donald Trump flew over the Washington DC protest in his helicopter as beneath him a crowd of around 15,000 flag-waving, baton-brandishing masked men and women massed.
On Saturday morning he tweeted: "Wow! Thousands of people forming in Washington (DC) for Stop the Steal. Didn't know about this, but I'll be seeing them! #MAGA."
Wow! Thousands of people forming in Washington (D.C.) for Stop the Steal. Didn’t know about this, but I’ll be seeing them! #MAGA
— Donald J. Trump (@realDonaldTrump) December 12, 2020
They listened to speakers including Rudy Giuliani's star witness in the Michigan fraud case, Mellissa Carone, who had to defend herself from accusations of being drunk during the hearing, and conspiracy theorist Alex Jones, best known for insisting the Sandy Hook school shooting was a hoax.
Jones was accused of inciting violence after declaring that Joe Biden "will be removed, one way or another".
On Monday the electors will meet in their respective states to declare the results of their state's election, and Joe Biden will officially become President-elect.
Saturday's protest in Washington DC - the second one called by pro-Trump forces - was peaceful until the evening, when once again members of the far-Right Proud Boys group in their distinctive yellow Gadsden flag banners fought with counter-protesters dressed in black and wearing ski masks.
Earlier on Saturday Enrique Tarrio, who heads the Proud Boys, was at the White House, posting a photo on his Parler social media account.
"Last minute invite to an undisclosed location…" he said.
Judd Deere, a White House spokesperson, said Tarrio "was on a public White House Christmas tour" and had not been invited.
Police did not enforce face mask rules or issue fines to those who ignored social distancing guidelines, even as the region faces an unprecedented spike in coronavirus cases. Dozens of DC police officers have tested positive in the weeks since the last pro-Trump rally in November. As of Friday, 94 remained in quarantine.
As night fell, scuffles broke out as Black Lives Matter signs were burnt and riot police desperately tried to keep the two sides apart.
"Both sides of the aisle hate you now. Congratulations," a Proud Boy shouted at the officers.
Fireworks were let off and people were pushed to the ground and stamped on.
Peter Newsham, chief of Washington DC police, made a brief appearance in the chaos, telling protesters: "We're doing the best we can."
He told the Washington Post that police units were deployed across downtown to keep the groups apart, and said smaller segments of people who splintered from larger gatherings seemed "intent on conflict".
Four people were stabbed outside of Harry's, a popular pro-Trump bar. The victims were taken to hospital and suffered possibly life-threatening injuries, said Doug Buchanan, spokesman for the DC fire department. It was not clear with which groups the attackers or the injured might have been affiliated.
Four more people were taken to hospital, including two police officers, and 23 were arrested.
— Donald J. Trump (@realDonaldTrump) December 12, 2020
On the other side of the country, in Washington state, a person was shot around 2pm local time on Saturday, and a suspect was detained.
In California's capital, Sacramento, five police officers were injured and six arrested.
Protests were also held in other communities around the country, including Atlanta, Georgia, another state where Trump's campaign has sought to overturn Biden's election victory, and in Mobile, Alabama.