Atlanta police Tuesday night had made 21 arrests of demonstrators and protesters of the decision of the St. Louis County, Mo. grand jury to not indict a police officer for the August shooting of Michael Brown.

Most of the arrests were for charges of obstruction or failure to disperse, though there was one felony weapons arrest, police chief George Turner said at a 10:45 p.m. briefing Tuesday.

Turner said most of the demonstrations were peaceful and said most of the arrests were of members of “splinter” groups. He said he was not aware of any injuries to demonstrators but said one Georgia State Patrol officer was injured in the neck by a flare shot by demonstrators.

Protesters broke a window at Meehan’s Irish Pub, at Wells Fargo Bank, and attempted to break a window at the Ellis Hotel, all on Peachtree Street downtown, Turner said.

One APD cruiser had a window smashed and a taxi also had a windshield smashed, he said.

“Our plans for the rest of the night are to be here,” Turner said, “and to make sure our city is safe. We will not leave our city unprotected.”

Atlanta police gave demonstrators an order to disperse after 9:30 p.m. when the marchers had reached the area of the Peachtree Street-Ivan Allen/Ralph McGill intersection downtown. About 100 or so marchers, according to an estimate from the Channel 2 Action News helicopter over the scene, had taken a circuitous route to the intersection where they were stopped by a line of police.

Turner said his force has been training for months in preparation for tonight and that for the most part, it was peaceful. It is unclear if the so-called splinter group broke off from the large rally at Underground.

“We thought it was important to allow individuals to express themselves peacefully,” Turner said. “We have a history of allowing people to express their constitutional rights.”

Staff writers Jon Gargis, Ernie Suggs and Jennifer Brett contributed to this article.

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