Andrew Miller, 46, of Church Road, Southbourne, near Emsworth, lashed out after John Lynch asked him repeatedly to stop flailing about and keep to his own seat.
A court heard leather-clad Miller had been 'shouting and whooping' and had stripped t
o his bare chest and removed his right false leg, which had an Alice Cooper motif on it.
He had also been thrusting his elbow into Mr Lynch.
When Mr Lynch got up to leave the seated area of Southampton Guildhall, Miller grabbed him by his hair as he walked past and dragged his head down.
Miller then punched Mr Lynch in his face up to 10 times, leaving him with severe bruising and a haemorrhage behind his right eye. Miller suffered a fracture to his right hand.
A jury at Southampton Crown Court found Miller guilty of assault causing ABH.
He was bailed to return to the court on July 8 to be sentenced.
Judge John Boggis QC told Miller: 'This was a nasty, unprovoked attack.'
Miller had denied inflicting ABH on Mr Lynch on July 25 last year.
>> Vote in our latest web poll.