The Harper government is throwing its weight behind a private member's bill that would give police the power to arrest anyone hiding their identity during a riot or unlawful assembly.

Conservative backbencher Blake Richards is proposing penalties of up to five years in prison or a fine of up to $5,000 for protesters who wear a mask or disguise.

The bill, Richards said in an interview, is designed to give police more power to prevent the kinds of riots that have caused so much damage, including the current student riots in Quebec, the Stanley Cup riot of last spring in Vancouver and the G20 protests in Toronto two years ago.

"Certainly I've heard of instances where it is legitimate that there might be reasons that someone needs to protest anonymously and this bill certainly still allows for that," said the second-term MP from Airdre, Alta., representing the riding of Wild Rose.

"I think it strengthens the right for peaceful protest. It's only when individuals engage in criminal activity or become violent where this law would apply."

Justice Minister Rob Nicholson announced Sunday that the Conservative majority formally supports the legislation, meaning it is all but assured of becoming law.

"Destructive and reckless behaviour damages communities and should not be tolerated," Nicholson said in a release.

Richards says the bill will allow police to step in and stopviolence and property damage if a protest turns ugly.

"Primarily I see this as a tool that will be a preventative one, one that will deter these situations from developing or from escalating in the first place," he said.

But some civil libertarians are concerned that the legislation will give police the power to break up peaceful protests, which are frequently filled with people in costumes, masks or even face paint that could be construed as concealing identity under the new law.

The provisions of the legislation specify that it only applies during riots or "unlawful assembly," a legal term in which police deem there to be reasonable grounds to expect a disturbance of the peace.

Françoise Boivin, the NDP justice critic, said the Official Opposition does not have a problem with the "concept" of the bill, but she says police already have the power to arrest and charge people intent on inciting a riot, and of using a mask to commit a crime.

Boivin argued that the additional law may simply muddy the water and give defence lawyers an opportunity to point out inconsistencies between the various statutes.

"We're still not really convinced of that factor, that for police it will be easier [to prevent property damage]," she said.

"What will be a legitimate excuse to cover your faces?"

The bill would create two classes of offence.

Those who incite a riot wearing a mask "without lawful excuse" face an indictable offence with prison terms of up to five years.

For those "who participate in an unlawful assembly while wearing a mask or disguise to conceal identity," the charge could be an indictable offence or a summary offence.

Under the summary offence, penalties range up to six months in jail and fines up to $5,000.