Whitehall was brought to a standstill for nearly three hours as a naked man straddled a statue in front of hordes of onlookers.
He was seen in a variety of poses on the statue of Prince George, the Duke of Cambridge, on Friday at times balancing precariously on the Duke's head.
The man was coaxed down shortly before 3pm and put into the back of a police van.
Officers were called at around midday to reports that someone had clambered on to the statue, and they believed he might have a knife.
A 100-metre stretch of Whitehall, which is near the heart of Westminster in central London, was cordoned off as emergency services tried to persuade him to come down.
The man was seen waving his arms around and was talking to crews on the ground as crowds of onlookers gathered at either end of the cordon. Scotland Yard said the man climbed down at around 2.45pm and was detained under the Mental Health Act.
David Lee, a civil servant who works nearby, said he had watched the naked man for more than an hour.
"We saw him dismount the horse, then he sort of disappeared for a bit, then I assume he went into the police van," Mr Lee said.
"It's amazing that someone can cause this much disruption. Perhaps the cold finally got to him, or the wind finally got to him."
