Eight people have been arrested after Greenpeace activists scaled Nelson's Column and other London monuments, and breached security at Parliament.
The environmental group put masks on statues, including the Oliver Cromwell statue in the grounds of Parliament.
The protest stunt aimed to highlight "dangerous and illegal" air pollution levels in the capital, the group said.
City Hall said it was taking "ambitious and comprehensive" measures to improve London's air quality.
Emergency services were called to Trafalgar Square in the early hours of Monday morning, and cordoned off the area around the 52m (170ft) high Nelson's Column.
Two climbers, named by Greenpeace as Alison Garrigan, 29, and Luke Jones, 30, had climbed the London landmark and fixed a white mask on the statue of Lord Nelson.
They both descended voluntarily at about 09:00, and were arrested by police on suspicion of criminal damage.
Speaking to radio station LBC from the top of the column, Ms Garrigan admitted she was "petrified of heights".
The experienced climber, who was one of six Greenpeace protesters who climbed the Shard building in 2013, told BBC Radio London the group had been working on plans for the stunt 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."
Other statues targeted by the group included the Sherlock Holmes statue near Baker Street Tube station, the Winston Churchill statue outside the Houses of Parliament, Eros in Piccadilly Circus and the Queen Victoria statue opposite Buckingham Palace.
London's Metropolitan Police said officers were called to Parliament at about 06:25 as a man and a woman, in hard hats and climbing gear, put a face mask over the statue of Cromwell.
They were arrested, along with two others at the scene, for breaching existing bylaws under the police reform and social responsibility act 2011, the Met said.
Two people were arrested on suspicion of criminal damage after scaling the Duke of Wellington Memorial in Hyde Park Corner at around 08:15.
The breach in security comes at a time when the UK terror threat level is "severe".
Greenpeace said its protest was to highlight the "public health crisis" of air pollution, and put pressure on whoever wins May's London mayoral election on 5 May to take action.
The group wants the government to focus on greener public transport, push electric vehicles and create clean air zones around schools and hospitals.
One or more legally-binding limits on air pollution levels have been breached in London every year since they were introduced by the UK and EU in 2005, Greenpeace said.
A study by King's College London last year suggested that 9,400 people a year died as a result of air pollution in London.
A spokesman for the Mayor of London said said "some of the most ambitious and comprehensive measures in the world" were being taken to help improve air quality.
The government said it had announced plans to improve air quality in cities in December, with plans for clean air zones in Birmingham, Leeds, Nottingham, Derby and Southampton by 2020.
A full list of London mayoral candidates can be found here.