One of them was "taken aback" as he heard the “thuds” of Sgt Nathan Legend’s clenched fist strike the prisoner’s cheek, he told a district judge.
Pc Thomas Richardson said the man they had arrested, Rennel Walker-Arthur, had not posed a threat to the officers at the time he was punched.
Sgt Legend, from Wolverhampton, had also failed to give all the regulatory warnings to the prisoner before using force, Birmingham Magistrates Court heard.
CCTV footage of the incident showed blood splattered on the mattress and floor of the cell at Perry Barr Custody Block where Mr Walker-Arthur was brought in the early hours of December 22 last year.
The alleged beating happened after he had been arrested in Birmingham’s Broad Street area for assaulting a police officer and causing criminal damage to a patrol car.
Sgt Legend, 31, was the custody officer who booked him in.
After answering questions about his health and any medical conditions, Mr Walker-Arthur was searched and removed his belt for the officers but refused to take off a ribbon attached to his jeans.
Sgt Legend explained to him that anything that could be used as a ligature had to be removed and when he still refused, the officer came around the custody desk and grabbed him around the neck from behind, dragging him to the floor.
Within seconds several other officers arrived on the scene and helped detain the prisoner, who was on his stomach on the floor.
Pc Richardson and another officer, Pc Peters, were holding down his legs while Sgt Legend, at his head, shouted at him to relax.
Giving evidence to the trial, Pc Richardson said that Sgt Legend then punched Mr Walker-Arthur. Initially he thought the prisoner may have tried to bite or spit at the sergeant to provoke such a reaction, he said.
But five seconds later he saw another blow land on Mr Walker-Arthur’s cheek.
'No imminent threat'
Pc Richardson said: “I was quite surprised at what I had seen because as far as I was concerned there was no imminent threat. I didn’t feel threatened.”
After the third strike, the constable said: “I was quite taken aback, I questioned whether it was needed. From what I saw, it wasn’t needed. I didn’t think it achieved anything.”
On being taken to the cell, it is claimed that Sgt Legend felled the prisoner to the floor using his leg, whilst he was handcuffed, leaving him unable to protect himself against injury on landing.
The officer said that Mr Walker-Arthur had been tensing his muscles in resistance and was shouting, but had not been violent.
He described Sgt Legend’s behaviour as “very robust treatment” and said he had been “alarmed” to see blood on the cell floor.
Sgt Legend pleads not guilty to two charges of assault by beating.
The prosecution is being brought by the Independent Office for Police Conduct.
The trial continues.