Police arrested 20 people after a Monday morning protest that saw activists scale the Parliament building in Ottawa and drape it with banners advocating government action to combat climate change.

Greenpeace protesters climbed the roof of Parliament's West Block on Monday morning. (CBC)Ottawa police said they arrested 19 of the protesters who had climbed on the building and one organizer on the ground.

They will likely be charged with mischief and possibly other offences, a police spokesman said. No one was hurt, and there were no altercations with police.

Christy Ferguson, a Greenpeace spokeswoman, said Mike Hudema, a Greenpeace campaigner from Alberta, was one of the climbers, but she said most of the demonstrators were volunteers.

"They really care about these issues, and they're willing to put themselves on the line," Ferguson said. "They're willing to risk arrest and face those charges in order to do something that they feel is important."

She said the group considers the protest a success since it has attracted attention to the issue and sparked conversation on the Hill.

"It really seems like the message has gotten out," Ferguson said. "They seem to be talking about it, and I believe Canadians are going to be talking about it. It has been pretty successful.”

Emily Rees, a bystander who saw the protest, said she thought the demonstration would help raise awareness about climate change.

"I think that it was good for them and good to get the word out and good to get awareness to more Canadians, but I do think it's a breach of security for them," Rees said. "I wouldn't be the one out there doing it, but I support them."

Police investigate how activists scaled building

The RCMP and Ottawa police are investigating how protesters in blue jumpsuits and white climbing helmets were able to scale the building undetected around 7:30 a.m. ET Monday and hang huge banners on it.

The activists rappelled from a wrought-iron fence ringing the roof of the building's West Block and unfurled banners that said, in English and French: "Harper-Ignatieff: Climate Inaction Costs Lives."

It's believed the activists who climbed the West Block used scaffolding that was set up at the back of the building. The RCMP, who are responsible for security outside of the Parliament buildings, wouldn't confirm how the activists got to the roof.

"The RCMP will be reviewing the incident, and we will take appropriate measures," said Cpl. Caroline Poulin, an RCMP spokeswoman.

"It's important for the citizens of this country to have access to Parliament. This is certainly something we want to maintain, but at the same time, we have to have appropriate security measures in place," Poulin said.

Security at Parliament Hill was increased in the wake of the September 2001 terrorist attacks in the U.S.

The results of the investigation will be watched carefully by Public Safety Minister Peter Van Loan, his office said.

RCMP, Ottawa police, the Ottawa fire department and city paramedics were all at the scene. When authorities did finally gain access to the roof, their first action was to take down the banners and signs.

The protest comes as world leaders and diplomats gather in Copenhagen for a United Nations conference on climate change. The aim of the meeting is to reach a new agreement on controlling greenhouse gas emissions to replace the Kyoto Protocol, which has its first commitment period end in 2012.

With files from The Canadian Press and the CBC's Alison Crawford