The crowd booed when Simon was asked to put his clothes on
A man participating in Antony Gormley's fourth plinth art project was made to cover up by police on Wednesday, when he stripped off during his slot.
The man, who gave his name as Simon, was fully clothed when he climbed onto the plinth in London's Trafalgar Square, but began to strip soon after.
The waiting crowd cheered, but booed once he put his underpants back on.
A spokeswoman for the One And Other project said it had been police who demanded the man get dressed again.
After about five minutes standing in the nude, Simon was told to put his underwear back on.
"Simon, you're going to have to put something on or they want you down," said one of the organisers. "Sorry mate. Some pants or something. Thank you."
Gormley, who is in Mexico and was unavailable for comment, had expected nudists on the plinth, the spokeswoman said, adding that those taking part must also act within the law.
Speaking on the project's website, Simon explained why he wanted to take part.
"Am just about to hit 50. Presently going through a difficult but long-anticipated separation.
"Feeling renewed, though. This event will serve to symbolise the beginning of a new age for me. Always wanted to be a sculpture."
A police spokesman said: "Officers were alerted to a naked male in Trafalgar Square this morning.
"The man was asked to put his clothes back on and immediately did so. No further action was taken."
The artwork is giving 2,400 people the opportunity to occupy the plinth for an hour each, 24 hours a day, for 100 days.
Simon is not the first person to have stripped off during the project, as last month a woman went topless and covered her breasts in paint.