Four Afghani teenagers were attacked out of the blue by five men and one woman on Monks Road on September 6.
Police say the attack took place at around 9.15pm near Lincoln College on Monks Road, by five offenders in their early 20s.
The offenders, all described as white, approached the four Afghani youths while they were walking along the road and attacked them.
One of the teenagers was badly injured. Munir Zafar (16) (pictured) was pushed to the ground and repeatedly kicked and punched by the group.
Zafar was taken to Lincoln County Hospital, where he was treated for a fractured skull, a fractured cheekbone, and bruising to his neck, shoulders, arms and chest.
Lincolnshire Police PC Michael Swaby described the incident as “an extremely nasty attack on a group of young men,” and is calling for witnesses.
One of the attackers was described as around 5ft 10in tall, and had a tattoo on his upper left arm.
The woman was about 5ft 4in tall with shoulder-length dark hair and a pierced lip.
“It is vital that anyone who witnessed it gets in touch with us as soon as possible to help us with our enquiries,” Swaby said.
“If you saw the incident, or you think you know any of the offenders then please call us as soon as possible.”
Police are urging anyone with more information to call 0300 111 0300, quoting incident number 487 of September 6.