St Annes Promenade was closed to drivers through the morning of Sunday, May 22 for the annual St Annes Triathlon, which involved more than 400 competitors swimming, cycling and running.
Event organiser Tim Armit said the marshal was treated in hospital after being hit repeatedly by a driver who left his vehicle to dispute being challenged over straying into the closed road area.
“It was totally unprovoked,” said Mr Armit. “This man leapt from his car, picked up a cone and hit a marshal on the back of his head concussing him and then started to punch him in the head as he went down.
"It is hard to believe, disgusting and I truly hope, like the football pitch invaders who assaulted a player, that this idiot is prosecuted and imprisoned.
"There is no justification and it was shocking for all those that witnessed it. The marshal stayed for an hour or so after the attack but then went to hospital.”
The Triathlon is now in its 14th year and on Sunday attracted almost 300 adults and 150 children aged seven and up, with competitors from as far as Scotland, Wales, Ken and Somerset.
The swimming aspect of the event takes place at St Annes YMCA Pool and the cycling and running over the whole promenade, which is closed from Fairhaven Lake to Todmorden Road throughout the morning.
A Lancashire Police spokesman said the assault is under investigation. Anyone with information should call 101 quoting log 531 of May 22.