Anti Brexiteers were ordered to take down a "provocative" poster and stop handing out leaflets at a family fete and fun day in Stony Stratford.
The European Movement Milton Keynes took a stall at the town's annual Riverside Fair this weekend.
A source for the fair said: "They said it would merely be for promoting European Collaboration. As it turned out, it was nothing of the sort and leaflets were being distributed to families attending and residents being approached rather like some clandestine US religious group."
It is understood complaints were made to police officers who were attending the fair.
"People had gone there for a fun day, not to see a platform for political debate and promotion," said the source.
He said the people running the stall were told to take down their provocative poster and cease handing out leaflets.
One member of the public said: "The Riverside Fair is a community lead annual event and it is very disappointing that those opposed to Brexit wanted to hi hack it for their own purposes.
He added: "'We are all up to our eyeballs in Brexit and should not be subjected to political campaigning on a day meant for families, especially children, to enjoy a peaceful day of fun".