(CNN) -- Houston's mayor and police department were on the defensive Friday, two days after graphic video came out showing several police repeatedly kicking and beating a 15-year-old burglary suspect as he lay on the ground.

An internal police investigation of the incident last March led to the firing of seven police officers, said spokesman John Cannon of the Houston police department.

Two successfully appealed and returned to their jobs, said Houston NAACP President D.Z. Cofield.

Five other officers were disciplined in other ways, Cannon said. And a Harris County grand jury indicted four of the officers this summer, based in part on the video.

Harris County District Attorney Patricia Lykos opposed the video becoming public and felt doing so might prejudice potential jurors and force the indicted officers' trials to be moved out of the county.

Quanell X, a local activist, got hold of the surveillance tape showing the scene outside a storage facility and gave it to the media.

He said he had every right to obtain the footage and make it public.

"I will show my people what they deserve to see, and let the public see what you don't want them to see," Quanell X said.

Mayor Annise Parker said the police leadership and city acted properly.

"I resent any implication that we were trying to hide the tape," she said.

After viewing the footage, Houston Police Chief Charles McClelland Jr. fired the seven officers and a grand jury called for misdemeanor charges against four of them in June.

Lykos told reporters Thursday there was not sufficient evidence to pursue more serious charges, such as aggravated assault.

"Without revealing what was presented to the grand jury, in order to have aggravated assault you have to have serious bodily injury or impairment or use of a deadly weapon," she said. "None of that was apparent in this case."

The tape, first shown Wednesday on CNN affiliate WTRK, shows the 15-year-old boy -- being chased by police and falling to the ground after being upended by a moving police car. He then falls face first and places his hands on the ground.

A disciplinary letter from McClelland, dated June 23 and posted online less than two weeks later by CNN affiliate HTRK, says that the boy had his hands behind his head and neck area, in an obvious position of surrender.

Then, the letter adds and the tape shows, Officer Raad Hassan "then ran toward (the boy) and kicked him a total of 15 times," then later kicked him more times in the groin area even after he "was handcuffed and no longer a threat."

Several other officers, repeatedly kicking and punching the 15-year-old, who barely moved the whole time.

Like Parker, the head of Houston's police union said the incident did not reflect on the make-up or usual activity of officers on the force.

"We have thousands of officers who do a great job every day and they're not involved in this," said Mark Clark, the union's executive director. "It's serious and it's a reflection on the department."

Cofield sharply criticized the police officers' actions as well as how civic leaders had handled the case afterward during a news conference Thursday. That includes officials' unwillingness to make the tape public.

"For us, what seems to be a tragedy (is) repeated one more time in Harris County," he said.

CNN's Divina Mims contributed to this report.