Photos of Chris Moyles wearing a red nose lit up the scrolling illuminated billboard around the top of the BT Tower in London Wednesday evening.
The surprise stunt was to raise awareness of the longest Radio 1 show attempt by Moyles and Comedy Dave, which has also started trending worldwide on Twitter.
The scrolling message read: "Good luck Chris Moyles and Comedy Dave for the longest show ever" and included pictures of Chris wearing a red rose. The event was live online and on the BBC Red Button.
Fellow presenter Greg James was at the top of the tower to announce the news to Chris live on-air just before 10pm.
Chris and Dave are aiming to break the previous record of 37 hours by Simon Mayo, and have said they want to go past 50 hours to reach the end of what would be his normal Friday morning show.
Just after 24 hours in, the show has currently raised £425.000 for Comic Relief.