The New York police department is facing renewed questions over the conduct of its personnel after a man alleged he was viciously assaulted without provocation by a group of uniformed officers.

Santiago Hernandez, 23, claims that he was kicked, punched and zapped with pepper spray by up to six officers in 157th Street in the Bronx on 18 August. A video of his arrest captured by a nearby security camera and published by the local TV channel ABC 7 shows the police initially frisking him, then handcuffing him and finally piling on top of Hernandez as he lay on the sidewalk while apparently hitting him with batons.

“They was taking turns on me. One kicks me, he steps back. Another one comes to punch me and he steps back. And another one comes and grabs my arm and hits me like 10 times with the baton. Another one comes and pepper sprayed me … they were taking turns like a gang,” he told ABC 7.

The new allegations, that are being investigated by the NYPD’s internal affairs section, come at a difficult time for the force and its high-profile commissioner Bill Bratton. A grand jury has been called to decide whether to bring charges against an NYPD officer involved in the attempted arrest in Staten Island of Eric Garner for selling loose cigarettes.

Garner, 43, died on 17 July as he was put in a chokehold – a procedure that has been banned in the force since 1993 – by officer Daniel Pantaleo, and was heard on video footage of the arrest saying, “I can’t breathe”. Last month a medical examiner found Garner had died of compression to his neck and chest, and declared his death a homicide.

Bratton, whose career has been built upon his reputation for improving relations between law enforcers and the public, has has been rattled by the Garner death. In January he took up the job of NYPD chief, which he previously held in the 1990s, vowing to end the hostility that had been generated by the widely criticized “stop and frisk” policy.

On Monday, Bratton faced two hours of questioning over the incident from the New York city council, focusing on the commissioner’s plans to retrain all 35,000 NYPD uniformed officers in how to make arrests with the minimum use of force. In addition to the training, the commissioner has also ordered the use of body cameras by some officers on patrol to record all their communications with individuals.

Lawyers representing Hernandez in the new controversy have told ABC 7 that they will be bringing a civil complaint against the police. Hernandez, who is currently on parole having served six years in prison for gang assault, said he was left bruised by his treatment. He insists that all he did in the moments before he was allegedly assaulted was to ask officers simple questions such as why he was being searched and handcuffed.

“I’m a person to ask questions. If I didn’t do nothing wrong, I’m trying to understand the reason,” he said.