London police said that the man, believed to be in his 30s or 40s, first climbed atop the massive bronze statue of the 19th-century duke of Cambridge around noon.
He climbed up and down the statue in the late autumn chill, at one point balancing himself on the duke's head.
The man eventually came down after police and emergency services cordoned off the area, which is home to several government buildings including the prime minister's official residence.
Scotland Yard said the man was detained under Britain's Mental Health Act.
Image: A naked man clambers all over a statue of George, Duke of Cambridge in Whitehall, London, Friday Nov. 23, 2012.