Environmental activists claiming to be from the environmental pressure group Greenpeace climbed the roof of the National Gallery on Tuesday evening to protest against oil firm Shell.

A crowd of hundreds of tourists and Londoners gathered to watch the eight protesters dressed in mountaineering equipment at the iconic location in Trafalgar Square as they unveiled a 22-meter long banner which read "It's no oil painting, save the Arctic."

The banner included the logo of the international oil company Shell, which was hosting a reception at the National Gallery on Tuesday night.

London's Metropolitan Police force could not tell Xinhua on Tuesday evening if the protesters would be arrested if they came down from the portico above the main entrance to the building.

The protesters, all wearing red jump suits and carrying sophisticated mountaineering equipment that allowed them to abseil up and down the early 19th century facade of the gallery from a portico above the main entrance on Trafalgar Square, appeared on the roof in the afternoon.

Police deployed six police vans and about two dozen officers. London Ambulance Service also sent two quick-response vehicles.

A Greenpeace activist broadcast on its twitter account, "We're on the roof of the National Gallery to ask Shell to keep out of the Arctic."

Also in Trafalgar Square was a giant, life-size electronic model of a polar bear deployed by Greenpeace as a publicity stunt, which walked among the crowds scaring some people who initially thought it was a real bear.

Greenpeace said on its website that it was protesting against Shell's oil spill response plan for drilling in the Chukchi Sea, which was last week approved by the U.S. government.