Seven of the intruders — aged 15 to 18 — were arrested on suspicion of causing grievous bodily harm after Chris Worthy’s birthday bash descended into horrifying violence.
Chris’s brother Stephen, 18, and father David, 53, needed hospital treatment for injuries suffered while trying to defend their home in Chippenham, Wiltshire, from uninvited guests.
A police spokesman said: “Even though it was advertised on YouTube, the gatecrashers were all just from the local area, a lot from the same school as the boy whose party it was.
“About 30 or 40 guests were invited but around 100 people ended up in the house.”
Chris, who goes to Sheldon School in Chippenham, threw the party on Saturday with the blessing of his parents Dave and Susan, on the agreement that no more than 40 people were invited.
But someone posted the event on YouTube, resulting in a mob of troublemakers sneaking into the house at about 7pm to help themselves to the drinks cabinet.
Mr and Mrs Worthy, at the time enjoying a quiet refreshment at a nearby pub, were called by eldest son Stephen, acting as a doorman, to say things were getting out of hand.
The Worthys rang police and returned to help officers clear the house.
But a group of youths appeared a short time later, and attacked Mr Worthy and Stephen. Stephen, a Bath University student, was airlifted to hospital with suspected spinal injuries but later discharged. His father suffered a broken nose.
The thugs, seven of whom were arrested, smashed windows at the house, which was left stained with blood.
Mr Worthy, an engineer, said: “It was absolute mayhem. One yob was swearing right in my face so I pushed him away and then all hell was let loose. I was punched in the face and hit on the head with something and fell backwards into a bush.
“Stephen came to help and was wrestled to the floor and kicked and punched. I think there were eight, nine or 10 people setting about us. We were trying to get away and one way or another we eventually managed to get inside and slam the door shut.”