Part of Whitehall in central London was cordoned off after a naked man climbed a statue.
Police said the man was clothed when he climbed the statue of Prince George, Duke of Cambridge, at lunchtime.
An officer at the scene said the man had snapped off part of the statue and waved it about before throwing it down.
About 100m of Whitehall, near Trafalgar Square, was closed off. The man came down voluntarily nearly three hours later.
The Grade II listed statue, which belongs to the nation but is in the care of English Heritage, is being assessed for damage, said a spokeswoman.
The cost of repairs is not yet known.
'It's an experience'
Crowds of onlookers gathered at either end of the cordon during the incident.
Eyewitnesses said the man could be seen waving his arms.
US tourist Lydia Kassa, 23, saw the man on the statue at 13:00 GMT while sightseeing.
"I just laughed," she said. "It's an experience. We didn't expect to see a naked man."
David Lee, a civil servant who works nearby, 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."
A member of staff at The Lord Moon on Whitehall said businesses in the area had lost thousands of pounds because the road being closed for so long.
"We should be absolutely full and it's killed us because they closed the road right off," he said.
He asked if "one naked man" required two fire engines, eight squad cars and four mounted police.
Scotland Yard said the man had been detained under the Mental Health Act.
A force spokesman said: "You apply the appropriate amount of policing for the situation.
"Obviously in a situation like this, public safety is paramount."