The group was able to get outside the tower through a window on the observation deck of the tourist attraction on Tuesday morning, Calgary police said. Once outside, the activists chained the window shut behind them.
They then hung nearly 160 metres from the ground to unfurl the banner — which read "Separate Oil and State" — on the north side of the tower in the city's downtown, where many oil and gas companies are based.
Members of the group wearing hard hats also blocked the base of the tower by putting up construction tape at the entrance.
The environmental group criticized what it called a "cozy relationship" between the oilsands industry and the federal and Alberta governments.
"While oil may run your car, it shouldn't run your government," Greenpeace activist Melina Laboucan-Massimo said in a news release. "Canada is not a petro-state and Big Oil should not be calling the shots and governments should not be ignoring the environmental destruction of the toxic tarsands."
Because of safety concerns, police negotiated with the protesters — who had set up a complex rigging system — instead of trying to physically remove them. The banner was taken away after about 30 minutes.