They managed to "break in" to Nottingham Prison while around 20 other protesters made a diversionary dash for the jail gates knocking over barriers and ignoring shouted warnings from the police.
Maggie Bucknall, a spokeswoman for the protesters, said: "We just wanted to show that if we could get in, [the paedophiles] could get out."
The three protesters, who supporters said had 16 children among them, used ladders to climb the walls before lashing themselves to stonework then unfurling banners opposed to the new paedophile accommodation unit.
The complex, which was expected to open on Wednesday, is still being transformed from offices in the prison hospital.
The first occupant of the bedsit-style accommodation is likely to be Lennie Smith, released this week and considered by senior detectives to be one of Britain's most dangerous child abusers.
Smith is being held in short-term secure accommodation in London after being released from prison on Thursday. The 44-year-old, a former rent boy who has been diagnosed HIV positive, was jailed in 1992 for abusing a six-year-old boy he was babysitting. During yesterday's demonstration, one of the protestors gave interviews to reporters from the prison roof using a mobile phone.
Mark Britten, 35, said: "What we wanted was to stop people coming in and out of the prison. We couldn't do anything that we had done before like chaining ourselves to cars so this seemed the next thing."
He described how he and two others had scaled the prison wall using ladders and had then had to shimmy for almost 200 yards along the top of the wall which is fitted with an anti-climb guard.
He said: "It was really quite unnerving and dangerous and it took us quite a long time to shuffle all the way along. We've got bleeding knees and elbows because it is fibreglass up there."
Mr Britten said abuse was also being shouted at them from inmates. "They want to know if they can get their visits. We're having to think about that because it was not our intention to stop them getting visits."
The three protestors gave themselves up after the fire brigade erected ladders to the prison roof. They were all arrested for trespass on prison property.Reuse content