Hundreds of people rallied in McKinney, Texas, Monday after video showed a police officer shoving a girl to the ground and pointing his gun at other teens.
Protesters said it was just another instance of unfair police violence against black people.
Demonstrators carried signs calling for justice and support for #BlackLivesMatter, the movement that began last year in response to the police killings of several unarmed black men.
"All lives matter," chanted protesters as they marched from an elementary school to the pool where the incident occurred.
Police blocked off roads in the Dallas suburb.
On Monday, Mayor Brian Loughmiller said the officer would remain on administrative leave while the incident was investigated.
Meanwhile, the pool was "temporarily closed."
"It is appalling that a man would draw a weapon in the presence of children," said David Lee, an author and former McKinney resident, told reporters Monday. He called the "rogue officers's" behavior "completely unacceptable" and urged parents to seek counseling for their children.
"Children wearing bathing suits were wrestled to the ground and handcuffed," Lee said. "Picture how traumatic an experience that is for your 14-year-old child."
Lee also demanded accountability from the McKinney police department. "Take a deeper look at the belief system of those you employ," he said. "Should the man lose his job, that's the least of what should happen."
Activists said they had filed an open records request for the 911 tape to see what the officer initially responded to.