West Yorkshire Police has admitted unlawfully arresting, assaulting and detaining a man who had come to the aid of two of its officers.
"Good Samaritan" Anwar Gillespie was helping the officers as they tried to arrest his neighbour, by calming down her son, when back-up officers arrived.
He was dragged outside, punched and kicked by the officers and locked in a cell for 24 hours in October 2007.
The force has now apologised and given Mr Gillespie £15,000 compensation.
A solicitor for the 39-year-old Bradford father of two said his role as peacemaker had been misinterpreted by the back-up team when they had arrived on the scene.
Iain Gould, who specialises in actions against the police, said Mr Gillespie had been pulled to the floor and kneed in the head and that an officer had pushed fingers into his eyes.
Mr Gillespie told police he had suffered trouble sleeping and was afraid to leave the house since the incident.
Mr Gould said: "Anwar Gillespie received rough justice for trying to help West Yorkshire Police, acting as a Good Samaritan.
"After the assault, false arrest, wrongful imprisonment and rejection of his complaint, the police wrongly denied all liability but made a derisory offer to settle his compensation claim.
"After seven further offers and two years of negotiations, Mr Gillespie's claim has settled and he has received the apology he deserved."
The force said lessons had been learnt.
A spokesman said: "On behalf of the chief constable I apologise unreservedly for the unlawful actions and for any pain, suffering, humiliation and upset this has caused."