Bright yellow paint was splattered on every one of the 70 stones in the King's Men circle in what site custodian Dohn Prout called a "coldly calculated attack".
But he vowed that the site would remain open to all visitors. "We cannot give in," he said.
The damage — which may end up costing as much as £100,000 in repairs — was discovered by a team of geophysicists working nearby, when they arrived last week.
The historical Pagan Federation has also condemned the attack — and offered a £1,000 reward for information leading to a successful prosecution.
Karin Atwood, a spokesman for the federation, said: "It is unbelievable.
"At least if it had been someone making a point you could attempt to understand — but it was so mindless, it is beyond comprehension. Our members feel hurt and outraged.
"In folklore terms, Rollright is a unique site, because it is the only one specifically associated with a witch," she added.
Mr Prout said he expected the repairs would take up to six months and cost the Rollright Trust, which owns the site, between £30,000 and £100,000.
"The estimate the restoration experts gave us was five-days-a-week for 25 weeks to finish, and they might not be able to get all the paint off, because it has got into all the cracks and holes. Hopefully it will eventually be worn off by the weather."
English Heritage restoration experts visited the site on Friday to begin assessing the damage and how best to deal with it, but their efforts are being made harder due to the rare forms of lichen which live on the stones. Some of it is thought to be more than 900 years old.
"It would be easier to get the paint off if we didn't mind getting rid of the lichen, but it is still growing and it is a living growing creature," said Mr Prout
"There may not be any other colonies so old in the country.
"Visitors to the site have been horrified. Many have come to look at the damage and been totally sickened by it. This is more than mindless vandalism, it is well thought out vandalism.
"They have splattered every stone and not got any on the grass or even left a can.
"It's not somewhere you would just have stumbled across, they would have had to come out specially to do it."
It is not the first time the Rollright Stones site has been the target of crime.
In 2000 a piece was chipped from one of the stones, and in May 2002 antique railings at the King's Stone were stolen.
Last May, the warden's hut at the King's Men circle was gutted by fire and a cash donation box ripped from a wall

l VANDALS daubed black paint on a wall at Chipping Norton war memorial last week. It has since been cleaned. Police ask anyone with information to call them on 0845 8505 505 or Crimestoppers anonymously on 0800 555 111.