Hundreds of far-right protesters were met by crowds of antifa, or anti-fascist, counter-demonstrators in Portland, Ore., Saturday, as President Trump singled out the latter for possible designation as a terrorist organization.

Confrontations between the two sides resulted in at least 13 arrests, according to Oregonlive.com.

Demonstrators in black clothing and with masks on their faces — a common antifa mode of dress — smashed windows of at least one shuttle bus carrying the far-right protesters, the news site reported.

“Major consideration is being given to naming ANTIFA an ‘ORGANIZATION OF TERROR,’” Trump wrote. “Portland is being watched very closely. Hopefully the Mayor will be able to properly do his job!”

It was the second time in a month the president floated the idea of slapping a terror label on the sometimes violent group.

In July, GOP Sens. Ted Cruz of Texas and Bill Cassidy of Louisiana introduced a non-binding Senate resolution that would do just that.

About 400 members of right-wing groups, including the Proud Boys, marched on the famously liberal city to protest the city’s inaction in the wake of an Antifa attack on conservative journalist Andy Ngo on June 29. Ngo was hospitalized with a brain hemorrhage.

There have been no arrests in the case, his attorney tweeted Saturday.

The local Rose City Antifa, billed as “the nation’s oldest active anti-fascist group” and known for its use of violent tactics in street confrontations, organized the two counter-demonstrations — but scores of heavily armed police officers kept the demonstrators far from one another. Three people were arrested, according to Portland cops, who installed concrete barriers to keep the rival groups apart.

“Frankly, it’s not helpful,” Mayor Ted Wheeler told CNN in response to Trump’s comments about the Antifa movement, which was formed in 2013.

“Adding to that noise doesn’t do anything to support or help the efforts that are going on here in Portland.”

But it was music to the demonstrators’ ears. “Go look at President Trump’s Twitter,” Joe Biggs, who organized the right-wing rally, told The Oregonian. “He said he’s watching Antifa. That’s all we wanted. We wanted national attention, and we got it. Mission success.”

Domestic terrorism is defined as criminal activity under the Patriot Act — but the law lays out no federal penalties for committing it. Instead, the actions of those classified as domestic terrorists are tried in state courts.

A “domestic terror” label, which the FBI applies to offenders after a crime has been committed, merely expands investigators’ powers. On Aug. 6, the FBI opened a domestic terrorism investigation into last month’s mass shooting at the Gilroy Garlic Festival in California, for example.

Foreign terror organizations are treated very differently. The State Department has designated about 50 groups as terrorist organizations since 1997 — most recently, the Islamic Revolutionary Guard Corps. Such designations carry a range of official sanctions and make it illegal for American citizens to support them.

Any attempt to apply such sanctions to domestic organizations would likely run afoul of First Amendment protections, experts say.

But several members of Congress — on both sides of the aisle — have launched efforts to expand the federal government’s ability to crack down on domestic terrorists.

Sen. Martha McSally (R-Ariz.) introduced a bill last week that would make acts of domestic terrorism a federal crime. Rep. Adam Schiff (D-Calif.) submitted his own legislation to the House on Friday that would do the same.

Additional reporting by Eileen AJ Connelly