The activists fixed the iconic monuments like the Nelson's Column and statues outside the British Parliament complex with symbolic gas masks on the faces of famous figures.
"We've climbed Nelson's Column in protest at dangerous air pollution... 40,000 lives are cut short by air pollution in the UK every year," Greenpeace tweeted.
Alison Garrigan, 29, and 30-year-old Luke Jones began climbing the 171-foot Nelson's column at 9:30 am today as part of their series of stunts, which led to eight arrests for public order offences andon suspicion of criminal damage.
"Monitoring shows that, if these statutes were real people, many of them would often be breathing dangerous, illegal air," said Greenpeace campaigner Areeba Hamid.
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"That's why we've given them face masks. Of course many millions of Londoners, including kids, are breathing that same air," Hamid said.
The group wants the UK government to focus on greener public transport, push electric vehicles and create clean air zones around schools and hospitals.
"At approximately 4.10 amtoday, 18 April police were called to a group of six protesters trying to climb Nelson's Column in Trafalgar Square," said aScotland Yard spokesperson.
"At around 9 amthe protesters came down from the column - two people were arrestedon suspicion of criminal damage. At approximately 6.25 amtwo protesters scaled the Cromwell Statue in Parliament Square. Shortly after 7 am, both voluntarily came down and were arrested," the spokesperson said.
Two other persons were arrested at the scene in connection with this incident. All four were arrested for public order offences.
Since 2005 London has broken World Health Organisation guidelines on safe air quality every year.