Greenpeace activists have scaled Nelson's Column, breached security at the Houses of Parliament and put face masks on 17 London statues in protest at "dangerous" levels of air pollution, BBC informed.
Two protesters climbed a fence and evaded security to put a mask on the face of the Oliver Cromwell statue within the grounds of Parliament.
The environmental group also targeted statues of Winston Churchill, and Queen Victoria outside Buckingham Palace.
Eight people have been arrested.
Emergency services attended the scene in Trafalgar Square and cordoned off the area around the 52m (170ft) high Nelson's Column.
Two climbers, named as Alison Garrigan, 29, and Luke Jones, 30, began scaling the London landmark at about 04:00 GMT. They both abseiled down safely at about 09:00 GMT, leaving a white mask on the statue of Lord Nelson.
Speaking to radio station LBC from the top of the column, Garrigan admitted she was "petrified of heights".
The experienced climber, who was one of six Greenpeace protesters that climbed the Shard building in 2013, told BBC radio London the group had been working on plans for over a month.
Asked about the burden the stunt placed on emergency services, she said: "I understand that some people might be frustrated by it. But we are highly trained, what we're doing isn't dangerous.
"This is about the air we're all breathing. In London the levels are both illegal and dangerous," she added.