A 16-year-old French student visiting Cambridge was punched, kicked and struck with a wooden pole in an unprovoked attack.
The boy was sitting with a friend on grass near the bus stop in Wadloes Road when four youths approached.
One attacked the victim while another youth struck him across the head and back with a wooden pole.
Police, who have only just released details of the attack on 27 July, said he suffered a serious injury.
He was treated at Addenbrooke's Hospital and later discharged.
Cambridgeshire Police has described the attackers as white and aged 15 or 16. The main attacker had brown hair and was wearing a dark top with light stripes on the sleeves.
Det Con Ellie Hains said: "This was an unprovoked and nasty attack on a boy who was minding his own business talking to a friend.
"I would like to hear from anyone who has information about those responsible or was in the area at time and witnessed anything that could be of use."
The attack happened at about 1930 BST.
The boy and his friend, who is also 16, then ran to a nearby McDonald's restaurant, where police were contacted.