Statues including Nelson (of Nelson's Column. In Trafalgar Square), Boudica and her daughters by Portcullis House, Winston Churchill in the Houses of Parliament and even Thierry Henry outside the Emirates Stadium were fitted with pollution masks.
"We want #CleanAirNow," Greenpeace tweeted, as activists strapped a gas mask to Nelson's face.
It being 2016, the Trafalgar Square protest - which involved two activists scaling Nelson's Collumn - was livestreamed on Periscope. You can watch it here:
Air pollution has been a huge problem in the capital this year - in January London exceeded its annual pollution target in a matter of days, while this time last year, a toxic smog cloud shrouded the City.
Earlier this year air quality monitoring firm Plume Labs launched a flock of pigeons fitted with "little backpacks", tracking air pollution in the city.
"Air pollution is a huge environmental health issue," said Romain Lacombe, PlumeLabs' chief executive, at the time.
This morning a spokesman for the Metropolitan Police said: "At approximately 4:10am on Monday, 18 April police were called to a group of six protestors trying to climb Nelson's Column in Trafalgar Square. At around 9am the protestors came down from the column - two people were arrested on suspicion of criminal damage.
"At approximately 6:25am two protestors scaled the Cromwell Statue in Parliament Square. Shortly after 7:00am both voluntarily came down and were arrested. Two other people were arrested at the scene in connection with this incident. All four were arrested for public order offences.
"At around 8:15am two people were arrested after scaling the Duke of Wellington Memorial in Hyde Park Corner. Both were arrested on suspicion of criminal damage.
"The protests at all three sites have now concluded."