LONDON — Six environmental activists scaled Western Europe's tallest building on Thursday to protest drilling in the Arctic by oil companies — and were arrested after reaching the top.

It took the female climbers 15 hours to reach the top of the 310-meter (1,017-foot) tower on the south bank of the River Thames in London.

Once they reached the summit, two of the activists unfurled a flag which read "Save the Arctic."

Shortly after, London police, who had been on the scene since dawn monitoring the situation, said the six protesters were arrested on suspicion of aggravated trespass.

John Sauven, executive director of Greenpeace U.K., said it was an honor to witness their "remarkable achievement" in the name of protecting the Arctic.

Greenpeace said the climbers were from Britain, Canada, Sweden, Poland, the Netherlands and Belgium.

It said the Renzo Piano-designed 72-story Shard had been chosen because it looms over the London offices of Royal Dutch Shell PLC, which has drilled for oil in the Arctic Sea off Alaska.

The skyscraper, which opened earlier this year, was kept open throughout the climb.