Campaigners from Greenpeace have breached security at the Houses of Parliament by scaling a statue of Oliver Cromwell and fix a mask to its face in an effort to highlight the UK's "filthy" air pollution levels.
Activists from the environmental group also climbed Nelson's Column in Trafalgar Square and have placed masks all over statues in London, including those of Winston Churchill, Sherlock Holmes, Thierry Henry and Eros in Piccadilly.
The breach in security at a time when the terror threat level is severe - the second highest - will come as an embarrassment to the authorities in Westminster.
Greenpeace said its activists carried out the stunt - spending five hours up Nelson's Column - to highlight the quality of Britain's air which it says is "so filthy it breaks laws".
Four people were arrested in connection with the incident within the grounds of the Houses of Parliament, the Metropolitan Police said.
A Met Police spokesman said: "The two people on the Cromwell statue have come down, they have been arrested.
Six protesters climbed Nelson's column and came down at 9am. Two were arrested on suspicion of criminal damage, police told HuffPost UK.
Greenpeace is targeting a total of 17 statues across the capital, including Queen Victoria opposite Buckingham Palace, in order to put pressure on whoever wins May's mayoral election to take action.
It sent out a list of statues that have been targeted so far, naming 13:
Campaigner Areeba Hamid said: "Monitoring shows that, if these statutes 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."
The Metropolitan Police said they were called at 4.11am on Monday after a group of six protestors were seen trying to climb Nelsons Column with a banner.
Campaigner Areeba Hamid said: "Monitoring shows that, if these statutes 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."
The Metropolitan Police said they were called at 4.11am on Monday after a group of six protestors were seen trying to climb Nelsons Column with a banner.
The environmental group has launched a petition calling on Prime Minister David Cameron to implement an action plan to improve the UK's air, saying pollution in the UK is responsible for cutting short 40,000 lives every year, Greenpeace claims.
Trafalgar Square trended on Twitter as Greenpeace rallied support with the hashtag #CleanAirNow.
