Videos showing the state of Sheffield's Old Town Hall show the 'desperate' need for investment, according to conservationists.
A series of clips of people exploring the disused building in Waingate were posted on YouTube at the end of October.
Headcam footage shows broken windows, water damage and graffiti in the once grand courthouse and council chambers.
In one video, the cameraman climbs up the clock tower and changes the time on the clock faces.
The Friends of Sheffield Old Town Hall group was set up to try to save the building. After watching the videos, chairman Valerie Bayliss said the need to redevelop the building was clear.
"The whole thing emphasises the desperate need to get the thing sorted," she said.
Property agent Fearnie Greaves has been trying to sell the Grade II-listed building for years. But last month director Tim Bottrill revealed its owner G1 London Properties Ltd was considering putting in a planning application to develop the site itself.
Mr Bottrill did not respond to a request for comment.
The Friends group has called for a 'financially sustainable mixed use'. Valerie said the videos highlighted a number of problems.
"It's very disappointing that people are still getting inside," she said.
"They are urban explorers who pride themselves on not doing damage. But if they can get in, then other people can."
On the clock hands being moved, Valerie said: "It's clear it's a very dangerous operation. There is always the prospect, because the clock is so rusted, that things will break."
Valerie said there was evidence internal windows had been 'systematically smashed' since the start of the year and there was fresh graffiti.
"In a way I admire the interest the explorers are taking in the building, but it is very dangerous. Some of the rooms have very dodgy floors because they are so wet," she said.
"It's just a scandal. What these kids are doing for us is showing us how much worse it's got."
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