A MAN has been attacked in an inner Sydney park after two boys demanded his mobile phone, then rejected it because it wasn't a smartphone.

One of the boys punched the man, 31, in the face and the other grabbed him from behind and assaulted him when he tried to run away from them in Redfern Park on Saturday night.

The boys, thought to be aged between 13 and 17, kicked the man while he lay on the ground and stole a sunglasses case from his backpack before taking off with four other teenagers.

The man needed hospital treatment for facial injuries.