A woman has been arrested on suspicion of criminal damage after a statue at Westminster Abbey was defaced, police have said.
Campaign group Fathers4Justice said it believed the woman was a campaigner for Stolen Children of the UK.
A Fathers4Justice spokesman said one of their campaigners had also been present but was not arrested.
Scotland Yard said they were called to the central London church just after 19:00 BST on Sunday.
A spokesman said: "At 7.05pm we were called to reports that a statue had been spray painted in Westminster Abbey.
"A woman has been arrested on suspicion of criminal damage and is being held at a central London police station. A man was also present but was not arrested."
The incident comes after Fathers4Justice said it was abandoning its five-year "attempted engagement with the political establishment".
The protest group has said it is refusing to engage with the government, police, courts, the judiciary or any other organisations involved in family law.
A spokesman for the organisation said on Sunday: "We want to see a transparent and open family court system."
Last week, Paul Douglas Manning, 57, a protester thought to be linked to Fathers4Justice, was charged with criminal damage after a photograph of a young boy was glued to John Constable's masterpiece The Hay Wain in the National Gallery.
Fathers4Justice campaigner Tim Haries, 41, from Doncaster, South Yorkshire, was charged with criminal damage after a painting of the Queen in Westminster Abbey was defaced.