Greenpeace activists suspended banners in an attempt to bring attention to the 'climate crimes of the tarsands.' (Greenpeace)

Sixteen Greenpeace activists were arrested early Sunday after spending 24 hours chained high up on smokestacks and a construction crane at a Shell Canada upgrader expansion site northeast of Edmonton.

The occupation started early Saturday when 19 activists stormed the under-construction upgrader in Fort Saskatchwan, which upgrades heavy oil into a lighter synthetic oil that can be refined into gasoline and other products.

Protesters from Canada, France, Brazil and Australia unfurled banners reading "Climate Crime" and "Climate SOS" to draw attention to an industry they say is dramatically increasing greenhouse gases.

The protest is the latest attempt by Greenpeace to shed light on the "climate crimes of the tarsands" in advance of the United Nations Climate Summit in Denmark in December.

Greenpeace said Sunday afternoon the 16 arrested remained in police custody. Charges may include break and enter, trespass and mischief, the group said.

Mike Hudema, a Greenpeace climate and energy campaigner, brushed off criticism from Premier Ed Stelmach that the protesters, who've launched three such protests in the last month, are being "coddled" while breaking the law.

Action highlights 'climate crimes'

On Sept. 16, Greenpeace activists chained themselves to two massive oilsands trucks in Shell's Albian Sands mine, north of Fort McMurray.

On Sept. 30, about 20 protesters were arrested after blocking work at Suncor Energy's open-pit mine in northern Alberta.

"This was another successful action to highlight the climate crimes of the tarsands," Hudema said.

"The addiction to oil that is fuelling tarsands development is leading to climate chaos. Already, hundreds of thousands of people are dying every year because of climate change. We have sent a message to world leaders that it is time to stop the destructive energy path of the tarsands and develop a clean energy future."

Shell Canada spokesperson Phil Vircoe said the site's operations were not interrupted because construction workers are only on the job Monday to Friday.

"Greenpeace has been targeting the entire oil and gas industry here, and they've been taking unsafe and outrageous actions to breach security at various sites over the last few weeks," Vircoe said.

Shell officials have said a safety review is being done to determine how the activists gained access to the site.

With files from The Canadian Press