These pictures show Greenpeace protesters putting gas masks on the faces of famous London statues

A group of activists from the environmental group Greenpeace attached gas masks to various London landmarks on Monday morning in a demonstration against the city's "dangerous" levels of air pollution.

Before the sun rose on the capital, a pair of protesters climbed to the top Nelson's Column - the iconic 171ft monument in Trafalgar Square - and put a gas mask on the face of Admiral Horatio Nelson.

Greenpeace told the Evening Standard that activists Alison Garrigan and Luke Jones began to climb the monument at around 4 a.m. on Monday morning. 

Garrigan was one of the six Greenpeace activists who climbed up London's tallest building the Shard in 2013 in protest against oil company Shell's plans to drill in the Arctic. The pair remained at the top of the monument before abseiling down shortly after 9 a.m.

 This is a picture one of the protesters took from the top of the monument.

Protesters also targeted statues of Queen Victoria, Sherlock Holmes, Oliver Cromwell, Sir Winston Churchill, Eros in Piccadilly Square, and former Arsenal FC footballer Thierry Henry.

This picture uploaded by Greenpeace UK's Twitter account, shows the moment an activist attached a mask to the Oliver Cromwell statue which stands outside the Houses of Commons.

Monday morning's demonstrations were part of Greenpeace's campaign to persuade Prime Minister David Cameron to develop a "clean air plan" for London and the rest of the country.

In its online petition, the group says "air pollution in the UK is responsible for cutting short 40,000 lives every year," and has presented a five-point action plan for protecting people from "dangerous and toxic" air - electric vehicles, clean air zones, car industry regulation, greener public transport, and less money spent on new roads.

Greenpeace campaigner Areeba Hamid told Sky News: "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."

A Parliamentary spokesperson confirmed that a "minor security incident" had taken place on the parliamentary estate that is being dealt with by the London Metropolitan Police, according to Sky News. 

A spokesperson for London Metropolitan Police told Sky that the two protesters who climbed Nelson's Column have been arrested, as were two other people linked to the incident.

BI contacted Greenpeace UK on Monday morning and was told the activists who took part in the protests are not yet available for comment.