President Donald Trump weighed in on the tense situation in Portland on Saturday morning, calling out the anti-fascist group known as Antifa on Twitter and suggesting support for labeling it a terror organization. He did not mention any of the right-wing groups, although both they and Antifa have a history of using violence against their opponents.

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

Many of the far-right demonstrators support a bill sponsored by Sens. Bill Cassidy, R-La., and Ted Cruz, R-Texas, urging Congress to identify Antifa, short for anti-fascists, as a domestic terrorist group.

The rally was the latest in a series of vocal and at times violent political demonstrations in a city where protest is a rich tradition but residents have grown increasingly weary of extremist saber-rattling on their streets.

For weeks, police and local politicians have been urging protesters not to show up at all and those who inevitably arrive to be peaceful. Officials and residents feared a melee like one in the city June 29, when a conservative writer was assaulted by black-clad protesters.

By Saturday afternoon, though, the tensions had not devolved into widespread violence. A few face-to-face confrontations broke out in the park as the rally began, but the far-right groups eventually moved behind a police line and were separated from the counterprotesters by a wide gap that officers worked aggressively to maintain.

At times, police intervened in occasional skirmishes, but any physical disputes were brief and limited to shoving and grappling.

Later, the right-wing protesters left the area and crossed the nearby Hawthorne Bridge to the east side of the city.

But members of the anti-government militia known as the Three Percenters and a few others stayed on the west side of the river, and Antifa members followed them through the downtown streets shouting at them to leave the city. At least two of those walking with the militia members appeared to have been hit with white substances resembling milkshakes.

Police said at least 13 people were arrested. Officers intervened in occasional skirmishes and seized flagpoles and shields from both sides.

A White House spokesman did not respond to an email seeking an explanation of Trump’s tweet, as well as why he chose to target Antifa and not others in the clashes.

But Trump has repeatedly sought to highlight incidents involving Antifa and has accused the news media of not giving the group’s tactics enough coverage. He has often highlighted them amid criticism of the rhetoric used by white nationalists.

Which groups are demonstrating?

Far-right groups congregated at the south end of the park Saturday morning. Some wore body armor and helmets, and at least one had a visible pocketknife and pepper spray, which he said were to be used “as a last resort.”

Among the other far-right groups were the Proud Boys, an all-male group whose members sometimes share racist or misogynist ideas, and who have fought with protesters before.

The most vocal promoter of the rally is Joe Biggs, who used to work for conservative conspiracy outlet Infowars. Biggs said he had organized the rally in response to the beating of conservative writer Andy Ngo in the clashes in June.

Many have blamed Antifa for the beating, which was captured on video. No one has been charged in connection with the assault, which police are continuing to investigate.

Rose City Antifa, which is based in Portland and is one of the oldest and most organized Antifa groups, has encouraged its followers to attend the rally.

Law enforcement officials established their presence early, seizing flagpoles and shields from both sides.

Portland police said they would use ‘whatever means necessary’ to curb violence.

“To those people planning to come and inflict violence in our city: We don’t want you here,” Ted Wheeler, the mayor, said in a recorded admonishment this month. He warned that police would use “whatever means necessary” to uphold the law.

The Police and the mayor have repeatedly said they would not target any political group but rather seek to stop violence perpetrated by people of any viewpoint.

Joey Gibson, the leader of another conservative group, Patriot Prayer, which has organized similar rallies in the past, turned himself in to the Portland police Friday after being charged with rioting in another clash in May. He appeared at the rally Saturday.

Two members of the Proud Boys are on trial in New York after being charged with attempted assault in an attack on people believed to be members of Antifa. The Proud Boys also hosted a free speech rally in Washington, D.C., in June, during which Antifa protesters clashed with police and some conservative demonstrators.

What would it mean to call Antifa a domestic terrorist organization?

The Antifa bill from Cassidy and Cruz is largely symbolic; there is no government list designating groups as domestic terrorist organizations, and the bill does not call on any federal agency to create one. It says simply that groups operating “under the banner of Antifa” should be labeled domestic terrorists.

The bill also asks the federal government to “redouble its efforts” to oppose domestic terrorism, including by white supremacists, and calls on the Senate to express “the need for the peaceful communication of varied ideas in the United States.”

The massacre in El Paso, Texas, earlier this month, in which a gunman killed 22 people, brought renewed calls for the creation of a law specifically outlawing domestic terrorism after police said the gunman had written a racist, anti-Latino manifesto.

While there is a federal crime outlawing “acts of terrorism transcending national boundaries,” there is no crime for domestic terrorism. People who are identified by police as domestic terrorists can be prosecuted for violating state or federal laws.

Why does this keep happening in Portland?

Opposing groups have faced off in Portland several times in recent years. Sometimes the protests turn violent.

Now used to the mayhem, residents and event planners in the city have prepared accordingly. A 5K run was moved from one side of the Willamette River to the other to avoid the protest, and police posted a map on Twitter identifying a dozen other events that they said would not be affected by the demonstration.

Rallies are so common in Portland in part because it is a hub for anarchists and radical political groups, drawn to the city’s reputation of upholding the rights to free speech and protest.

“We’re proud of our defense of these core American values,” Wheeler, the mayor, said in the video earlier this month. He repeatedly stressed, however, that the city did not want groups coming from out of town to stoke tensions.

This article originally appeared in The New York Times.