The Royal Canadian Mounted Police have arrested 16 Greenpeace protesters, including 13 who had chained themselves to a Shell oilsands upgrader plant in northern Alberta.

Police moved in around 5:30 a.m. Sunday, 24 hours after the activists scaled three smokestacks and a construction crane at the Scottford upgrader expansion near Fort Saskatchewan.

"It was a peaceful resolution to what could have been a very dangerous situation," RCMP spokesman Cpl. Darren Anderson said.

Four protesters climbed down on their own accord after hours of negotiations with the RCMP. But nine others refused to move.

RCMP and Edmonton Police Service members used ropes and harnesses to scale the tall smokestacks and arrest the nine remaining protesters.

"These police officers are specially trained in rappelling and use of ropes and have some background in mountaineering training as well," Anderson said.

Most of the protesters then agreed to climb down using their own gear, but two of them refused and had to be brought down by police.

In total, 16 Greenpeace protesters were arrested during the two-day incident.

Each has been charged with mischief and break and enter, and all have been released. They are scheduled to appear in provincial court on Nov. 4.

Three protesters were arrested Saturday as they attempted to gain access to the site.

Officials at Shell have announced they are conducting a safety review to determine how the protesters managed to get onto the site.

Shell spokesman Phil Vircoe said his company was concerned about "Greenpeace's unsafe and confrontational tactics. This placed their own safety at risk and also the safety of others who were on site at that time and throughout this process."

After their arrival Saturday, the protesters, who hail from Canada, France, Brazil and Australia, put up yellow banners reading "Climate Crime" and "Climate S.O.S." They said they were protesting the oil and gas industry for contributing to climate change through the emission of greenhouse gases.

A video clip posted on Greenpeace Canada's website Saturday showed a man standing in the dark in front of a large industrial facility.

"Right now we've climbed some kind of chimney that's (under) construction," the man says. "It's kind of chilly."

Alberta Premier Ed Stelmach called the protesters "lawbreakers" who were "putting people on the job site at risk."

Shell officials echoed that sentiment, saying Saturday they hoped to meet with any activists thinking of targeting its facilities.

"We reiterate our wish that Greenpeace and all other who have concerns with our activities would talk to us face to face instead of using confrontational and unsafe tactics," the company said in a statement.

The protest came after about 20 Greenpeace activists were arrested at a Suncor oilsands plant near Fort McMurray, Alta. a couple of weeks ago.