A security breach in Parliament saw two protesters scale a statue of Oliver Cromwell and fix a mask to its face as part of a campaign by Greenpeace.
The environmental organisation has targeted a series of statues in London, including Nelson's Column and Eros in Piccadilly Circus as part of a protest over air pollution.
Four people were arrested in connection with the incident within the grounds of the Houses of Parliament, the Metropolitan Police said.
Police were called at 6.25am as a man and a woman, in hard hats and climbing gear, put a face mask over the statue of Cromwell.
The two protesters, wearing helmets bearing the Greenpeace logo, came down from the statue and were met by waiting police.
A Met Police spokesman said: "The two people on the Cromwell statue have come down, they have been arrested.
"Four people in total have been arrested in connection with what has been going on."
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 a total of 17 statues were being targeted across the capital, including Queen Victoria opposite Buckingham Palace, in order to put pressure on whoever wins May's mayoral election to take action.
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."