A former Avon and Somerset Police constable has been jailed for 14 weeks after admitting a charge of common assault against a 15-year-old boy.
The charge against Adam Kitchener was made following an Independent Police Complaints Commission (IOPC) investigation into the arrest of a teenager in Bristol city centre.
The assault took place in Marsh Street at about 04:00 on 31 March.
Kitchener appeared before Bristol magistrates.
The assault happened when police were called to attend a robbery in the Broad Quay area of Bristol.
The boy was subsequently arrested in Marsh Street.
Supt Simon Wilstead, head of professional standards at Avon and Somerset Police, said: "The assault came to light following an internal complaint made by a colleague who witnessed the assault and a further complaint from the boy's family.
"Mr Kitchener was suspended and the matter was referred to the Independent Office for Police Conduct who carried out an independent investigation, resulting in him being charged with assault.
"This was a wholly unnecessary and unprovoked assault and on behalf of Avon and Somerset Police, I'd like to apologise to the victim and his family."
He added that although Kitchener had since resigned, a misconduct panel hearing would take place in due course.