Two men from the Fathers4Justice campaign group have climbed on to a Buckingham Palace roof to protest father's rights.
Martin Matthews, 48, and Bobby Smith, 33, used a ladder to reach the roof after another campaigner caused a distraction outside the palace, a spokesman for the group told ITV News.
They scaled the roof of the Queen's Gallery, a public art gallery at Buckingham Palace, and held a banner which read: "I am Harry's dad".
The Metropolitan Police were called at 4.30pm and described it as an "ongoing situation".
No arrests have been made.
James Dennis, a third activist, also tried to scale the roof but is thought to have been stopped by security.
Mr Smith dressed as the Sesame Street character Elmo when he stood against David Cameron in the Prime Minister's constituency of Witney, Oxfordshire, in the general election.
He previously scaled Westminster Abbey on Father's Day.
