The security breach happened at a Vancouver Board of Trade event at a downtown hotel, where Harper participated in a friendly question-and-answer session aimed at promoting his government's economic agenda.
Roughly a minute after Harper was welcomed to the stage, a man and woman walked behind the prime minister holding signs attacking the Conservatives' environmental record. One sign said Climate Justice Now, while the other featured a dark line crossed through the phrase Conservatives Take Climate Change Seriously.
The protesters were immediately taken off the stage and out of the room, prompting Harper to quip, "It wouldn't be B.C. without it," drawing laughter and applause from the audience.
A group affiliated with Brigette DePape, the former page who walked onto the Senate floor holding a Stop Harper sign during a 2011 throne speech, immediately claimed responsibility, saying the stunt was designed to criticize Canada's environmental policies.
The event took place in a large hotel ballroom, where hundreds of people gathered for a breakfast event featuring the prime minister.
While reporters and TV camera operators were forced to have their equipment examined by a police dog, there appeared to be little in place — such as a security checkpoint to check credentials or ID — that would have prevented anyone from just walking into the room.
Indeed, one of the two protesters, local activist Sean Devlin, said he and his colleague weren't stopped or approached by security at any point before they walked on stage.
"We just happened to be wearing black dress shirts and black pants and black aprons that we bought at Value Village, and we simply walked into the hotel," Devlin said in an interview.
"As soon as we entered the room, people seemed to be treating us like we were supposed to be there. It was quite easy."
The outfit cost about $7, he said.
Devlin was thrown down a small flight of stairs as he was forced off stage, while the young woman he was with was escorted away on foot.
They were handcuffed and brought into a kitchen area to be questioned, said Devlin, but were quickly released without being charged.
The prime minister's office says it doesn't comment on security matters and referred all calls to the RCMP.
Cpl. Lucy Shorey of the RCMP said the force would be examining what happened, but she declined to discuss the incident in detail.
"The RCMP takes the matter very seriously," said Shorey.
"We are currently reviewing the incident to decide what action needs to be taken to ensure the safety of the prime minister. Obviously, we can't discuss specific details on security measures."
Board of trade CEO Iain Black, who was on stage beside Harper, said event staff were in the room keeping a watch for anyone without one of the lanyards that were handed to ticketholders, but he said his group deferred to the RCMP when it came to security decisions.
Black said the incident happened so quickly that it was pretty much over by the time he fully processed what was going on.
"I didn't really react, because there wasn't a lot of time," Black said in an interview.
"I looked at the prime minister, and he didn't look rattled at all. ... He's clearly got a lot of confidence in the guys around him, so when he didn't react, I didn't ever really feel threatened."
Harper made several stops in the Vancouver area on Monday, but he did not field any questions from reporters.
The event was the first stop on a tour designed to highlight the Conservative government's economic record, particularly the recent agreement in principle with the European Union on a free-trade agreement.
But the trip also took Harper into the epicentre of antipathy towards the government's environmental policies, particularly as the Northern Gateway pipeline and other natural resource projects make their way through various stages of review.
Harper was careful not to take a position on the Northern Gateway pipeline during the question-and-answer session, insisting his government will wait for the approval processes to unfold.
A joint federal review panel approved the project, with conditions, in a report released last month, but the federal government has not yet said whether it will give the pipeline the go-ahead.
"The government's role is not to endorse particular pipelines or particular energy business projects," he said.
"Our job as a government is to set up proper processes of evaluation and scrutiny. We have a process of environmental review. The government will follow that process and take appropriate decisions, as it always has."
The proposed $8-billion pipeline, which would move Alberta crude to Kitimat, on the B.C. coast, has prompted fierce protests from First Nations groups and environmentalists.
B.C. Premier Christy Clark has publicly condemned the current pipeline proposal, saying it does not meet a list of five conditions the province has set out, including strict measures to protect the environment.
However, the provincial government has also suggested the door isn't closed yet.
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