A 21-year-old man has been arrested on suspicion of trespass after a person was seen climbing The Shard building in central London.
Metropolitan Police officers were called to the 1,017ft-tall (310m) skyscraper at 05:38 BST.
London Fire Brigade and the ambulance service also attended, and a number of cordons were put in place.
Two other men were detained on suspicion of causing public nuisance, the Met said.
Footage posted on social media appeared to show a figure scaling the exterior of the landmark, one of the biggest buildings in Europe.
Previous attempts to climb the 87-storey tower in Southwark include a group of protesters from Greenpeace in 2013.