OTTAWA — Ottawa police laid mischief charges against 20 Greenpeace activists Monday after a brazen security breach on Parliament Hill that one senior Liberal senator called "hugely embarrassing" for the government.

Colin Kenny, the Liberal chair of the Senate committee on national security and defence, said Monday it's a good thing the protesters were armed with climbing ropes and not rifles.

"We're very lucky it was just Greenpeace," he said. "If there were 19 people arriving with AK-47s, you could have a hell of a mess on your hands."

Kenny questioned whether there are enough security officers stationed at each building entrance and whether police could get to the Hill quickly enough if there were a serious incident.

Activists wearing blue coveralls, climbing harnesses and hard hats made it to the roofs of both the West Block and Centre Block and unfurled protest banners in a high-wire demonstration designed to grab the attention of federal politicians on the opening day of the UN climate change summit in Copenhagen.

Greenpeace spokeswoman Jessica Wilson said 19 activists arrived and were in place on top of the buildings between 7 and 7:30 a.m.

But she wouldn't say how they got there.

"In terms of the hows, that's not something we tend to talk about mostly because it distracts from the whys," she said.

"I appreciate the intrigue, but you can't always pull the curtain back on the magic show."

Police learned there were people on the roof of the West Block and Centre Block at around 7:15 a.m., but couldn't say exactly how they got there.

"That's what is being investigated right now to make sure appropriate actions are being taken by the RCMP and other security to make sure it doesn't happen again," said RCMP Cpl. Caroline Poulin.

In a Greenpeace video shot from the Centre Block roof, two RCMP officers appeared to stroll by the building without noticing anything amiss as the demonstrators prepared for their publicity stunt at about 7:30 a.m. The video also appeared to show a ladder against Centre Block.

The first arrests came at about 8:20 a.m. after a banner was displayed on Centre Block that read, in French, "Stop the tarsands."

The drama at the West Block played out longer. Six protesters who rappelled down the building's face one by one unfurled their banners by about 9:15 a.m. The banners read: "Harper/Ignatieff: Climate inaction costs lives."

Within minutes, firefighters used a bucket truck to remove the banners and then picked up the six activists.

One Greenpeace organizer was plucked off the sidewalk and arrested.

Ottawa police Const. Alain Boucher said the protesters — who ranged in age from 20 to 63 — were charged with mischief and could face fines or jail time, depending on whether or not they have criminal records.

Scotty Greenwood, executive director of the Canadian-American Business Council, who was attending an international trade conference on the Hill Monday, noted that protesters would never be able to scale buildings in the heavily guarded U.S. capital.

"I really noticed the differences between Canadians and Americans as I came to the building this morning," he said. "Here in Canada, I would just observe that there were . . . lots of emergency personnel, fire trucks, paramedics etc., there to make sure they were OK should something happen, ensure their safety. In our place it would have been snipers. No kidding."

About two dozen spectators, most on their way to work, gathered on the sidewalks outside Parliament Hill.

Marion Sewell-Sneyd applauded Greenpeace's actions.

"Good for these people," said the retired high school teacher. "Sometimes to draw attention, you need to take drastic measures. It's hard to get a message out to Harper."

Sewell-Sneyd, 66, said she, like most Canadians, is worried about Canada's stance on environmental issues, particularly the oilsands.

The environmental group is known worldwide for its dramatic protests.

In October, Greenpeace staged three protests at Alberta oilsands sites owned by Suncor and Shell. ,

In one of the occupations, the activists chained themselves for days to four conveyor belts at an oilsands site near Fort McMurray.

With files from Linda Nguyen