Last Saturday, just before midnight, Javier Payne and a friend stood handcuffed outside a hookah store on a street corner in the Bronx. The teenagers had been arrested, accused of assaulting a 49-year-old man who refused to give them cigarettes.

At one point Javier, 14, shouted an obscenity at a nearby police sergeant.

Then, officials and witnesses said, the sergeant grabbed Javier and pushed him hard against the window of the hookah store, shattering the glass and badly cutting Javier’s head and body.

Witnesses and officials said it did not appear that the sergeant intended for the glass to break. Regardless, the episode has become the first serious accusation of police brutality since Commissioner William J. Bratton took over with a mandate to improve the image of the department.

On Saturday, a week after the episode, Javier, in a wheelchair and with a white bandage on his head, appeared with family members and city officials at a news conference led by the Rev. Al Sharpton, who pledged to “monitor and pursue this case.”