Fifty protestors lay at the foot of the first-century amphitheatre, spelling out the words "Active Now," while eight climbers unfurled a giant banner across the monument reading "Copenhagen: Make History Now!"

"Climate change and world peace are inextricably linked," Francesco Tedesco, head of energy and climate issues at Greenpeace Italy, said in a statement.

"This is a destabilising time bomb for world peace," he warned. "Unless world leaders act now to cut greenhouse gases, the planet will face mass disappearances of animals and plants, vast migrations and famines."

Greenpeace said US President Barack Obama and other leaders "now have a chance to guarantee us all a sustainable, peaceful future and to make history by doing so."

Twelve days of climate talks kicked off in the Danish capital Monday, tasked with forging a deal to curb greenhouse gases, and help poor countries cope with the consequences of global warming already underway.

Greenpeace has staged a number of demonstrations in various countries to draw attention to the Copenhagen conference and the threat of global warming.

In Ottawa on Monday 20 activists were arrested after scaling Canada's parliament and unfurling two enormous banners calling for action on climate change.

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