Christchurch police are door knocking in Hornby for witnesses to an assault on an 11 year old boy.

Detective Senior Sergeant Mark Warner says the the boy's told them he was pulled off his bike around 6.15 last night and assaulted by a man while he was cycling along Vanguard Drive.

The boy managed to break free and cycle home

"We're taking this very seriously, and we are going to be conducting our enquiries in an attempt to identify that man who has been described as the offender."

The man's described as Caucasian, around 50-years-old with a grey goatee, prominent pot-belly and 170-centimetres tall.