The US president repeatedly took aim at left-wing counter-protesters who faced off against right-wing extremists in the Oregon city on Saturday.
“Major consideration is being given to naming ANTIFA an ‘ORGANIZATION OF TERROR’,” he claimed on Twitter, while remaining silent on the neo-nazi groups the activists took to the street to confront.
Mr Trump later shared a tweet depicting anti-fascist activists as violent thugs “looking to hurt and injure anyone in their way”.
“Everywhere the group ANTIFA (Anti-First Amendment) goes, violence and chaos follows,” wrote Fox News commentator Dan Bongino in the post retweeted by the president.
Joe Biggs, one of the organisers of the right-wing rally, celebrated Mr Trump’s renewed animosity towards the anti-fascist movement.
Hundreds of anti-fascist activists took to Portland’s streets to confront far-right groups, including members of the Proud Boys, which calls itself a “western chauvinist” fraternity, "patriot" militia the Three Percenters, and the white supremacist American Guard.
Police made 13 arrests and seized metal poles, bear spray and other weapons, although authorities managed to largely avoid clashes between the rival groups.
Mr Trump, who infamously blamed "both sides" after an anti-fascist demonstrator was murdered by a neo-nazi in Charlottesville two years ago, had fuelled tensions in Portland by comparing anti-fascists to terrorists and warning he was watching the city "very closely”.
His intervention was described by Portland’s mayor as “frankly, not helpful”. Ted Wheeler told CNN: “This is a potentially dangerous and volatile situation, and adding to that noise doesn’t do anything to support or help the efforts that are going on here in Portland.”
Leaders of the right-wing groups have vowed to keep returning to Portland, seen as one of America’s most liberal, as long as anti-fascists remain active.
"The path forward for Mayor Wheeler is simple, free your city from the grip of Antifa, take direct and meaningful action," said Proud Boys chairman Enrique Tarrio in a statement.
Mr Wheeler said the right-wing groups were not welcome in the city and warned the raly was the sign of "a rising white nationalist movement".
"Portland being a very progressive community is always going to be at or near ground zero of this battle," he added.