McKinney Police Corporal Eric Casebolt, who is white, had been placed on administrative leave pending an investigation of how he appeared to target black youths at the disturbance on Friday in the city about 30 miles (50 km) north of Dallas, an incident that has raised fresh questions about racial bias in U.S. policing.
"He came into the call out of control and as the video shows was out of control during the incident," McKinney Police Chief Greg Conley told a news conference. Casebolt tendered his resignation on his own, Conley added.
In the video, Casebolt is seen shouting obscenities at black youths in a multiracial crowd, shoving an African-American teenage girl, briefly pointing his gun at black youths and throwing the black 15-year-old girl in her bathing suit to the ground, putting his knees on her back.
"I had 12 officers on the scene, and 11 of them performed according to their training. They did an excellent job," Conley said.
The seven-minute video, viewed 9 million times on YouTube as of Tuesday morning, shows officers responding to the incident, which police said started when scores of young people attended a party with a disc jockey at a community pool and refused requests to leave.
Casebolt, a 10-year veteran once named the department's patrolman of the year, was questioned by authorities on Monday. He has not spoken publicly about the incident.
The role of race has divided the fast-growing city of about 150,000, which has attracted young families seeking affordable and spacious housing, good schools and a sharply lower crime rate than in Dallas.
In 2014, McKinney was ranked number one in Money magazine’s annual ranking of the Best Places to Live in America. It had a median household income of $80,000 in 2012.
For some, Casebolt's actions in targeting African-Americans, who make up about 10 percent of McKinney's population, pointed to bias, while others said he acted properly in helping to break up a party that had gotten out of hand and blame the teenagers for not obeying the police.