Three people are being questioned after obscenities were written on a Countryside Alliance stand at the Labour Party conference in Brighton.
Simon Hart said the Countryside Alliance was used to vandalism
A man and two women were arrested on Wednesday morning on suspicion of defacing the stand with marker pens.
It was part of a display at the Hilton Metropole Hotel, which is within the conference's security cordon.
A spokesman for the Countryside Alliance put the cost of the damage at an estimated £3,000.
"It is going to cause us a certain amount of inconvenience as we are supposed to be taking this stand to the next Conservative Party conference in Blackpool on Saturday," said Simon Hart.
"The police haven't charged anyone yet so we don't know who they are or why they chose our stand."
Police said the two women were arrested in "a drunken state" at 0230 BST at the Hilton.
The man was held at 0730 BST at a different hotel in Brighton.
A Sussex Police spokeswoman said there had been "no security breach" during the overnight attack on the stand, because all three suspects had clearance for the hotel and the conference itself.
Mr Hart said Countryside Alliance displays were vandalised on three nights out of four at the 2004 Labour conference in Brighton.