CAMBRIDGE, Mass. -- The mayor of a Massachusetts city is calling a video that shows a police officer punch a black Harvard University student several times while he's pinned to the ground "disturbing." Cambridge Mayor Marc McGovern said in a statement Sunday that the police department has assured him the altercation on the video is being investigated.

McGovern says "Cambridge affirms that Black Lives Matter, but it must be true in practice as well."

Police say officers were called Friday night after receiving reports of a naked man standing on a traffic island. A woman reported the man had thrown clothes in her face, and Cambridge Police received about six other 911 calls about the man, according to a department statement.

Police identified the man as 21-year-old Selorm Ohene. In the statement, Cambridge police chief Branville Bard said officers they learned from his friends that he had taken narcotics "which could have a hallucinogenic effect when ingested."

Get Breaking News Delivered to Your Inbox

Bard said they made "numerous attempts" to try to calm Ohene down but he was hostile when officers tried to talk to him. Officers say they saw him "clinching both fists" and taking steps towards officers, so one officer grabbed his legs to bring him to the ground, the statement said.

Police say the man resisted arrest once on the ground and the officer struck the man to gain compliance. Onlookers can be heard on the video released by the department telling police not to punch him and that they're being recorded.

Bard said it took three Cambridge Police officers and a transit officer to place Ohene in handcuffs. They say he was transported to the hospital for an evaluation and spit a mixture of blood and saliva at an EMT while en route.

According to the Harvard Crimson, members of the Harvard Black Law Students Association who saw the arrest have questioned the police version of events that the man was making aggressive movements, saying the officers tackled the student without provocation.

In the statement, police commissioner Bard said his officers undergo extensive training in crisis intervention, mental health response and de-escalation techniques. He said the man was displaying "erratic" behavior, likely because he had ingested drugs.

"When individuals are in a time of crisis — whether it's due to drug use, mental illness or other variables — this can create great complexities in an officers response," Bard's statement said.

Bard said the use of force was required to make the arrest, but the "primary concern" he's addressed from the community is that one of the officers punched the suspect five times while he was on the ground.

Bard said once the suspect was on the ground, he "contorted his body in a way that pinned his arms under his body and officers were unable to handcuff him" and an ongoing struggle ensued.

"To prevent the altercation from extending and leading to further injuries, particularly since the location of the engagement was next to a busy street with oncoming traffic, the officers utilized their discretion and struck the individual in the mid-section to gain his compliance and place him in handcuffs," the statement said.

In a statement, Cambridge city manager Louis DePasquale called the arrest "an opportunity to reflect on lessons that can be learned from this incident" and said he is awaiting the outcome of an internal review by the department's Professional Standards Unit.

According to the Harvard Crimson, university president Drew Faust called the arrest "profoundly disturbing" but cautioned not all is known because the review is still pending.

Ohene was arrested on charges including disorderly conduct and assault.

In a statement, the Harvard Law attorneys who are representing Ohene said the mathematics student is "currently recovering from injuries sustained during his encounter with the Cambridge Police Department."

"This has been and continues to be a trying ordeal for Selorm and for his family," said the lawyers, professor Ronald S. Sullivan Jr. and professor Dehlia Umunna, director and deputy director of Harvard Law School's Criminal Justice Institute. "Although there has been significant extrajudicial commentary on Selorm's case, we do not intend to litigate these matters in the media."

Sullivan and Umunna said "the video speaks for itself."