The tower on top of Glastonbury Tor in Somerset has been covered in purple paint and had graffiti daubed on its flagstones.
The National Trust, which owns and maintains the Tor, said it was "dismayed" after spending more than £500,000 recently on its restoration.
The Trust said the southern face of St Michael's Tower had been targeted during Sunday night.
The group Fathers 4 Justice said one of its sympathisers was responsible.
Andrew Mayled, National Trust Somerset property manager, said: "This is a terrible case of vandalism to the Tor.
"Glastonbury Tor is a Scheduled Ancient Monument of national importance and this wanton vandalism is the equivalent of desecrating Stonehenge.
"We hope we will be able to begin removing it on Monday without damaging the monument"
"As a charity, we will now need to expend our valuable time and resources attempting to restore the Tor to its former appearance.
"First impressions suggest that the still-wet paint used to vandalise the Tor is a water-based paint and we hope we will be able to begin removing it on Monday without damaging the monument.
"However, access to the top of the Tor is very difficult for vehicles and we will need to look at how we get pressure-washing equipment to the top of the Tor."
Last year, the Trust completed a £540,000 five-year restoration plan of the tower and Tor. Avon and Somerset Police said the incident had not been formally reported to them, and that they could not therefore comment.
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