Several Greenpeace members climbed the outside of the Olympic Stadium tower on Thursday to protest the federal government's decision to buy the Trans Mountain pipeline project.
The five climbers, who were equipped with cables and harnesses, ascended partway up the 165-metre-tall structure, which is built at a 45-degree angle.
They then unfurled a banner as a message for Prime Minister Justin Trudeau, with the sign reading, Stop Pipelines, Don't Dirty Our Money.
They later descended without incident and were arrested by Montreal police.
Greenpeace spokeswoman Loujain Kurdi said the climbers wanted to send a message to Trudeau that "climate leaders do not build pipelines.''
In a phone interview, she said Ottawa's decision to spend $4.5 billion to take over the Trans Mountain project goes against its international commitments to reduce emissions.
"Instead of spending $4.5 billion of taxpayer money on one pipeline, Prime Minister Justin Trudeau could help us build three million electric rechargeable stations,'' she said.
"For even less than that, for only $3.2 billion, he could fulfil the promise he made last December and ensure that every Indigenous community across Canada has access to drinking water.''
She added Trudeau still has time to back out of the deal, since the government and the company have given themselves until this coming Sunday to seek a third-party buyer for the pipeline.
Police cordoned off the area below the tower, and tourists and employees were barred from the stadium until around 11:30 a.m.
Montreal police spokeswoman Caroline Chevrefils said officers arrested four women and three men between the ages of 26 and 40.
Two women were arrested on the ground at around 9:30 a.m., whereas the five climbers were stopped after descending from the structure at around 11 a.m.
All seven will be charged with conspiracy and mischief over $5,000 and will be released on a promise to appear, Chevrefils said.
The organization carried out a similar action earlier in July when several climbers dangled under Vancouver's Ironworkers Memorial Bridge for 36 hours to create what the organization called an "aerial blockade,'' according to a Greenpeace news release.