In a series of Twitter messages before midnight ET, the president also blasted counter-protesters as "Human Radical Left garbage," retweeting a video that showed a man being sucker-punched and kicked on a Washington street earlier in the day.
WARNING: GRAPHIC VIOLENCE
Trump accused Antifa activists of returning under cover of darkness to cause mayhem, claiming the pro-Trump crowd had sent the leftists running "for the hills" during daylight hours.
The president also blasted Washington Mayor Muriel Bowser, claiming the Democrat was not "doing her job," and urged the city's police force to "get going -- do your job and don't hold back!!"
Trump also accused the "Silent Media" of ignoring the Washington clashes, retweeting a message from Republican U.S. Rep. Lee Zeldin of New York, who alleged a "total media blackout" as leftists launched "abhorrent" assaults against Trump supporters.
The evening saw scattered confrontations and escalating violence as police reported at least 20 arrests as they tried to keep opposing groups separated.
Earlier in the day, leftist counter-protesters turned out in opposition to pro-Trump groups rallying to question the results of the Nov. 3 presidential election, which showed Democrat Joe Biden denying Trump a second term in the White House.
The president himself even visited the demonstration -- traveling through the area in his motorcade.
Earlier Saturday, the march was largely peaceful. But as the sun went down, snippets of scenes shared on Twitter showed chaos had overtaken areas of the nation’s capital.
WARNING: GRAPHIC LANGUAGE AND VIOLENCE
Authorities said one male demonstrator was stabbed multiple times near the intersection of New York Avenue and 11th Street, and sustained injuries that were not considered life threatening. It was not immediately clear which side he was on.
Daily Caller contributor Matthew Miller shared what he described as an "all out brawl between Proud Boys and Antifa in the streets."
The Proud Boys is a conservative group known for occasional brawls in other cities, including at least once in New York City and at several demonstrations with Antifa members on the West Coast. One of the group's leaders was stabbed during another protest in Washington during post-Election Day demonstrations.
People wearing the group's colors, black and yellow, cropped up in numerous photographs and videos from the demonstrations.
The video from Miller showed two groups trading blows in an intersection as onlookers chanted profanities.
Anti-Trump demonstrators allegedly hurled eggs at Trump supporters -- or stole signs, hats and banners, and lit them on fire. Videos show them shouting at families, sucker-punching people in the street and harassing an elderly woman carrying a Trump flag.
City police said they’d made at least 20 arrests – three of which involved assaults – but a total count wasn’t expected until Sunday morning. At least two police officers were injured and authorities confiscated seven firearms, according to a city spokeswoman.
They closed off streets and part of Washington's downtown, already home to boarded up stores and offices.
In some places, police physically stood between groups of Trump supporters and counterprotesters, chanting or carrying signs and banners emblazoned with Antifa and Black Lives Matter slogans, to keep them separated.
Numerous videos on Twitter showed shouting matches and violence on streets and sidewalks.
One showed an altercation escalate with a mob of alleged “BLM-Antifa thugs” swarming and punching a man from behind in an intersection. Moments earlier, he attacked someone holding a megaphone. He shoved people away until someone carrying a “Trump/Pence Out Now!” sign finally knocked him out with a sucker punch. When he rose a few moments later, there appeared to be a gash on his face and a lot of blood.
Another angle of the attack showed someone stomping on his head while he was down. He reportedly was taken to police after the video ended.
Another showed "a group of purported Antifa members" burning a flag near Black Lives Matter Plaza, just north of the White House.
And yet another showed what appeared to be fireworks going off in the outdoor seating area of a restaurant with a crowd of onlookers just a few feet away.
A National Park Service permit for the rally was issued for up to 10,000 people, although several thousand more appeared to show up.
The Associated Press contributed to this report.