Up to eight activists from environmental protest group Greenpeace climbed on to the roof of the National Gallery in London.
The campaigners said they were protesting against oil firm Shell's plans to drill for oil in the Arctic.
They unfurled a 40m (131ft) long banner from the roof bearing the picture of an oil rig and reading 'It's No Oil Painting'.
Scotland Yard said the protesters came down from the building at 20:15 GMT.
No arrests were made.
Greenpeace itself describes what took place on its website:
So our climbers have made sure that guests at the National Gallery are met with an unexpected picture when they arrive; a short while ago, they evaded security and are preparing to unfurl a huge banner with the words “It’s no oil painting”. Our climber Hannah is tweeting from the rooftop using the hashtag #SaveTheArctic.
"No arrests were made". One sort of wonders what would have happened if some ordinary people would have done the same thing and started tweeting from the rooftop of the National Gallery?