
A Greenpeace activist abseils down Nelson's Column statue in Trafalgar Square, central London, after attaching a breathing mask to it on April 18, 2016.

The protests were carried out across London, adding masks to famous statues to highlight the problem of air pollution.

"Nearly 9,500 people a year are dying because of air pollution alone (in London)," Greenpeace campaigner Paul Morozzo tells CNN.

"Air pollution is an invisible problem and we wanted to make it visible before the mayoral elections (in May), because it's important that the mayor puts it as one of their main things to deal with," Morozzo tells CNN.