Two 16-year-old boys were hospitalised after three car loads of youths stopped and attacked them as they walked home.
The assault happened late at night in Cox Green Road, Maidenhead, Berkshire.
Both boys were punched and kicked and one of them was hit several times with a weapon that police believe may have been an iron bar.
They were taken to Wexham Park Hospital to be treated for injuries including a fractured jaw, cuts to the head, broken noses and cuts and bruising.
Attackers 'knew' victims
The incident happened at about 2200 BST on Friday.
"A small red car, possibly with a different coloured bonnet, and possibly a Renault, was seen pulling up near the boys just before the incident," police said.
Insp Mark Millward, of Thames Valley Police, said: "We believe this was not a random attack but involved people who were known to each other.
"I would urge anyone who saw this assault or knows those involved to contact police as soon as possible."