PORTLAND, Ore. (AP) — Portland police detained a crowd of at least 100 protesters Friday night at a demonstration, halting the march minutes after it started, the Oregonian/Oregonlive.com reported.

The mass detainment shortly after 9 p.m in the Pearl District appeared to an example of kettling, a police tactic of surrounding a crowd and containing people within a perimeter, the newspaper reported.

In a tweet and at the scene, police said they were detaining everyone within the boundary for the “investigation of a crime.”

“You are not free to leave. You must stay where you are and comply with officers’ lawful orders,” the Portland Police Bureau said in a tweet.

Adam Costello, a livestream videographer, told the newspaper that two windows were broken during the early stages of the demonstration. Costello said that may have prompted police to set up the perimeter.

In January, a group of protesters carrying signs against President Joe Biden and police marched in Portland on Inauguration Day and damaged the headquarters of the Democratic Party of Oregon, police said.

Portland has been the site of frequent protests, many involving violent clashes between officers and demonstrators, ever since the police killing of George Floyd in Minneapolis in May. Over the summer, there were demonstrations for more than 100 straight days.

Mayor Ted Wheeler has decried what he described as a segment of violent agitators who detract from the message of police accountability and should be subject to more severe punishment.