McKinney, with around 150,000 people, is a fast-growing, mostly middle-class suburb with deep racial and economic divisions. In 2009, according to an article in The Atlantic, the city settled a lawsuit in which it was accused of hindering the construction of affordable housing in the western part of the city, which is more white and more affluent.

The pool party took place on the west side, in a neighborhood that residents said is usually marked by friendly relations among black, white, Hispanic and Asian residents.

In a statement, the Police Department said officers arrived at the pool around 7:15 p.m. on Friday, responding to a call about a “disturbance involving multiple juveniles at the location, who do not live in the area or have permission to be there, refusing to leave.” The department, the statement added, received “several additional calls related to this incident advising that juveniles were now actively fighting.”

Mayor Brian Loughmiller said in a statement that he was “disturbed and concerned by the incident.”

On Monday evening, a diverse group of several hundred people gathered at an elementary school for a peaceful protest, waving signs reading, “Don’t tread on me or my kids” and “Stop Police Brutality.”

Speakers used a bullhorn to call for the respect all citizens, and for sensitivity training for the McKinney police.