Police arrested 20 Greenpeace activists after a demonstration in which the protesters scaled two Parliament Hill buildings early Monday morning, in a highly embarrassing security breach.

Those detained include 19 protesters and an organizer who remained on the ground. They have all been charged with mischief and are to be released soon, according to an Ottawa Police Service news release.

The 20 individuals were from all over Canada, but none were from Ottawa. They ranged in age from 20 to 63.

The RCMP, as well as Ottawa police, are investigating how the protesters managed to gain access to the buildings.

Five protesters scaled Centre Block, while another 14 scaled West Block, where six managed to rappel down the front of the building and hang banners that said "Harper-Ignatieff: Climate Inaction Costs Lives."

By mid-morning, firefighters removed the last of the protesters, some of whom were dangling by ropes over the side of West Block.

Greenpeace spokespeople would not say how the protesters breached Parliament Hill security to gain access to the buildings.

RCMP spokesperson Cpl. Caroline Poulin also had few details about the incident during a news conference with reporters at the scene, except to say that the incident is "still under investigation."

But protester Mike Houdema said why the protesters scaled the buildings is more important than how.

"My mission this morning is to send a strong message to the Canadian government on the opening day of the international climate change meeting in Copenhagen that climate inaction costs lives, that the longer that our government delays, the longer that it blocks progress, means the greater the number of people that will die or become displaced due to the climate crisis," Houdema said in a telephone interview from atop West Block with CTV News Channel.

"The UN global humanitarian forum has already told us that 300,000 people will die this year alone due to climate change. So the time for inaction has really passed and it's time for our government to start acting because millions of lives really do depend on it."

Tom Quiggin, a security intelligence expert, said the security on Parliament Hill "has been a problem for years."

"There's no one in charge of security on Parliament Hill and there's no one responsible for security . . . directly," he told CTV News Channel's Power Play. "There's the RCMP, the House of Commons police, the Senate police and on the perimeter, there's the Ottawa police."

The incident began unfolding around 7:30 a.m.

About an hour later, the five protesters on Centre Block were taken away in handcuffs by police after unfurling a banner that read, "Stop the Oil Sands."

By mid-morning, all of the protesters had been removed from the buildings, some by firefighters on ladders mounted to fire trucks.

The breach is shocking, given that security on Parliament Hill had been beefed up in the wake of the September 11, 2001 terrorist attacks in the United States.

During a noon-hour news conference, Poulin would not answer reporters' questions about whether the incident indicates government buildings are susceptible to similar protests, or even a terrorist attack.

Meanwhile, the demonstration shows that Greenpeace is willing to ramp up the audacity of its stunts in order to get across its climate-change message.

The group waged protests throughout the fall at a number of facilities in Alberta's oilsands region.

In October, RCMP arrested 16 protesters, including 13 who had chained themselves to a Shell oilsands upgrader plant in northern Alberta.

In that incident, activists scaled three smokestacks and a construction crane at the Scottford upgrader expansion near Fort Saskatchewan and unfurled banners that read "Climate Change" and "Climate S.O.S."

In September, 20 Greenpeace activists were arrested at a Suncor oilsands plant near Fort McMurray, Alta.

Greenpeace spokesperson Christy Ferguson said protests like Monday's on Parliament Hill are part of the organization's mandate of non-violence to transmit their message.

"All of our activities, all of our actions are always peaceful, but they're very powerful," Ferguson told CTV News Channel. "It's part of a long tradition of civil disobedience where people stand up to government, they stand up to people in power, they stand up for what they believe in and they do so in a peaceful manner but still a very powerful manner, and that's core to what we did here today."