The climbers were spotted on the domed building, which was originally built for Expo 67, at about 7 a.m. ET Wednesday.
A spokesperson for the group said the climbers will unfurl a banner with a "clear" message at about 8:30 a.m.
The group is asking federal Minister of Foreign Affairs John Baird to get involved and clear the jailed protesters.
Dressed in bright orange jackets and pants, the activists attached themselves to the dome with industrial climbing gear and harnesses.
Their spokesperson said the activists are experienced climbers.
Emergency crews are on the ground, but no rescue efforts have been attempted.
2 Canadians among jailed Greenpeace activists
A group of activists, including Quebecer Alexandre Paul and another Canadian, were detained in September while protesting Arctic oil drilling.
Paul's parents joined the protest at the Biosphère on Wednesday to call for their son's release. His mother, Nicole, said she was pleased with the protest.
"I am very happy about this," she said.
Paul is scheduled for a bail hearing on Thursday.
Six of the jailed foreign Greenpeace activists were granted bail by Russian courts on Tuesday, rulings that lawyers said will allow them to return home pending trial for a protest against Arctic offshore oil drilling.
The other Canadian, Paul Ruzycki of Port Colborne, Ont., was among those cleared for release.Suggest a correction