A teenager was punched to the floor by attackers thought to be using a knuckle duster before his bicycle was stolen.
Four men in a black Honda pulled up in a car park next to Pargate Chase, in Norden, Rochdale, and approached the victim.
The victim was punched until he fell to the floor, causing injuries to his face.
Police believe a knuckle duster may have been used in the attack.
Three of the attackers fled in the Honda, which made off along Edenfield Road, in the direction of Rochdale.
The remaining attacker rode off on the victim's bicycle, a black Transition Covert 2011.
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Police are appealing for any witnesses to the robbery, which took place on Friday, August 16.
Three of the attackers are described as white, but no description is available for the fourth man.
One offender was around 5ft 10 tall, of stocky build and aged 18 to 21-years-old with a Rochdale accent.
He was wearing a bright blue hooded anorak.
Another of the attackers is described as 5ft 10 tall, of 'chubby' build and also aged 18 to 21-years-old.
He had short curly light brown hair and was dressed in a brown or green coloured Nike hoody.
Anyone who has any information about the robbery or current whereabouts of the mountain bike should call police on 0161 856 4646, quoting incident number 2304 of 16/8/2019.
Details can also be passed on anonymously to the independent charity Crimestoppers on 0800 555 111.
