Save Fenton Town Hall say they are protesting because the Ministry of Justice (MoJ) is selling the building.
They say a World War One memorial is at risk of being destroyed if the building is sold.
However, the government has said "a legal covenant" means any buyer would have a duty to preserve the memorial.
The group said they wanted to see the building preserved for community use and were prepared to stay there all night.
The magistrates' court, which was built in 1886 as a town hall, closed in December 2012 as part of government plans to shut 93 courts in England and Wales in a bid to save Â£41m.
The Great War Memorial, which contains 498 names of soldiers from Fenton, is built into the wall of the court.
An MoJ spokesman said: "We are aware of the issue at the court. We understand the police are in attendance and the protest will be a matter for them."
Some 30 campaigners have occupied a former town hall and magistrates' court in Stoke-on-Trent.