Northumbria Police said the 25-year-old was arrested on suspicion of carrying out a stabbing and taken to South Shields station on 22 June.
He suffered bruising to his face and the Independent Police Complaints Commission (IPCC) is investigating.
All custody staff - including managers - have been moved to other jobs while the probe is carried out.
Chief Constable Sue Sim said she expected the "highest standard" of behaviour from her staff.
"The safety and protection of the public is of paramount importance and that includes anyone brought into custody," she said.
"We routinely deal with thousands of prisoners each year who come into custody and almost all of these pass through without any incident.
"I expect the very highest standards from all officers and staff dealing with the public in any capacity and will not tolerate anything that falls short of this.
"Rest assured we will take any appropriate action necessary in respect of this issue."
Vera Baird, Northumbria Police and Crime Commissioner, said: "The IPCC, I expect will conduct a thorough inquiry.
"I am grateful to the chief constable for informing me of this incident - I commend the swift and decisive action she has taken."
Two police detention officers have been suspended over claims that an arrested man suffered bruising while in custody.