Two men dressed as Batman and Robin are holding a rooftop protest at the Royal Courts of Justice in central London over the treatment of fathers in the family courts.
The stunt is said to mark the start of "civil disobedience" protests
Fathers for Justice campaigners Jolly Stanesby, from Ivybridge, Devon, and Eddie Goreckwi, from Romford, Essex, climbed onto the roof of the building on The Strand just after 0400 BST on Tuesday.
The men, who are calling themselves "the caped crusaders for justice", want to highlight the group's campaign to ensure divorced fathers are given access to their children.
They are believed to be equipped with food, bedding and clothes and plan to stay on the roof for a whole week, F4J spokesman Glen Poole said.
The stunt marks the beginning of "civil disobedience" protests against the court system across the UK, he added.
A Scotland Yard spokesman confirmed that two men dressed as Batman and Robin were on the roof and officers are trying to persuade them to come down.
Mr Stanesby last month took his campaign to the top of a 120 feet crane in Exeter, where he stayed for three days.
During his protest, he told reporters he was a registered child-minder with no criminal record but was not permitted sufficient access to his daughter.