Two Greenpeace activists have scaled Nelson's Column to attach a gas mask to the famous statue to highlight air pollution in Britain.
Alison Garrigan, 29, and Luke Jones, 30, began climbing the 171ft (52m) central London landmark at 4am on Monday as part of a morning of stunts in protest at "dangerous" air quality levels.
Other environmental activists attached gas masks to other statues across the capital, including those of Oliver Cromwell and Winston Churchill outside Parliament.
In total 17 other statues were masked, including Queen Victoria opposite Buckingham Palace, Eros at Piccadilly Circus and Thierry Henry outside Arsenal FC's Emirates Stadium in north London.
Ambulance crews and police officers stood at the bottom of Nelson's Column, which was roped off as the activists stood at the top of the statue.
Greenpeace campaigner Areeba Hamid said: "Monitoring shows that, if these statues were real people, many of them would often be breathing dangerous, illegal air.
"That's why we've given them face masks. Of course many millions of Londoners, including kids, are breathing that same air. Kitting everyone out with face masks is not the solution; instead we need to see real political action from the new mayor.
"We need a clean air zone covering a large part of the city. Whoever wins the election has to stop the talk and start the action."
Figures show that nearly 10,000 people die prematurely from air pollution in London every year.
Greenpeace is calling for an expanded clean air zone which will ensure that many more Londoners can have healthy lungs and clean air.
The current proposal for an ultra low emissions zone - to come into force in 2020 - only offers protection from dangerous air to residents and commuters in zone 1.
Ms Hamid said: "At schools across London children are being forced to breathe illegal, dangerous air. Londoners need greener and affordable public transport, along with air pollution alerts and an efficient and adequate system to measure air quality.
"The next mayor should begin a consultation on a clean air zone immediately after the election".
A total of 40,000 premature deaths are attributable to air pollution across the UK.
Apart from London, Birmingham, Leeds, Liverpool, Cardiff and Edinburgh among others, are likely to continue to breach legal limits of air pollution until at least 2020, Greenpeace said.
A parliamentary spokeswoman said: "There was a minor security incident on the parliamentary estate that is being dealt with by the Metropolitan Police Service.
"Parliament is working closely with the police on their investigation and we cannot comment further while this is ongoing."