A 17-year-old German exchange student is in a serious condition in hospital after he was attacked by a group of teenagers, leaving him with severe head injuries that required emergency surgery.

The victim, who is thought to be originally from Lebanon, was airlifted to a London hospital where he underwent specialist surgery. He is understood to be in a coma.

Six teenagers were arrested after the incident in Canterbury on Thursday evening. Police have appealed for witnesses to come forward and say a possible racist motive for the attack is one of their lines of inquiry.

The victim’s parents are settled in Germany but are not EU citizens. When they approached British officials seeking an emergency visa to visit him after the attack, they were told it would take up to five days to process.

The MP for Canterbury, Rosie Duffield, approached the home secretary, Sajid Javid, and urged him to expedite matters so the boy’s parents could visit as soon as possible.

It is understood Javid intervened personally on Saturday to arrange an emergency visa. The boy’s mother arrived in the UK on Sunday morning, three days after the attack.

In exceptional or compassionate cases such as this, it is understood the Home Office has the power to make arrangements to waive visa requirements outside normal processes.

The attack is being dealt with by the Kent and Essex serious crime directorate.

Officers were called to the centre of Canterbury at about 6pm on Thursday after reports that a teenage boy had been assaulted. Police officers and the South East Coast ambulance service attended and the teenager was airlifted to hospital where he remains in a serious condition.

A 16-year-old boy, a 16-year-old girl and two 15-year-old boys, all from Canterbury, and a 17-year-old boy from Margate were arrested as part of the investigation. A 44-year-old man of no fixed address was arrested on suspicion of assisting an offender. All were later released on bail pending further inquiries.

A further suspect, a 17-year-old boy from Canterbury, was arrested on Saturday evening and remains in custody.

Duffield said: “We are appalled by this vicious attack. My team worked on this yesterday and I approached the home secretary to request that visas for family members were expedited. The home secretary responded positively to this request.

“We want to make it clear that Canterbury is open to visitors and that we respect and admire other nations. We are so upset about this attack and want to offer as much support as we can to the victim’s family. Following an attack of this kind, we want to focus on what we have in common, rather than on the politics of division.”

Jamie Bell, a solicitor at Duncan Lewis who has provided assistance in the case, said: “In response to this awful assault, Kent community have demonstrated their disgust towards this vicious attack. Rosie Duffield MP should be commended for doing everything to support the family in their time of need and sending the message that Kent is a warm and welcoming place.”

Police have urged any witnesses to the attack to call 01843 222 289, quoting reference 46/108995/19. Alternatively, witnesses can call Crimestoppers in Kent, anonymously, on 0800 555 111.