When police in Montreal kettled student protesters, they were using a controversial crowd control method that has led to complaints of human rights violations, but one that has also been called the "least intrusive and most effective" tactic available to officers.

The Montreal incident came almost two years after police in Toronto boxed in hundreds of people for hours in the rain during the G20 summit in Toronto.

What is kettling?

The basics of a kettling operation: Police corral people in a central area (panel 1) and surround the crowd (panel 2), allowing those inside to leave only at police discretion (panel 3). (CBC)

Kettling is a police tactic to control crowds where officers surround a group of people on all sides. In some instances, police direct protesters toward a predetermined location. As the crowd grows, the police presence tightens around them.

Police control access to the location and decide how to allow people to leave, perhaps through a predetermined spot.

Where does the term come from?

The use of the word kettle in this instance is based on the German word "kessel" — a cauldron, or kettle — to describe an encircled army about to be annihilated by a superior force, according to a BBC report tied to uses of the crowd-control technique in the U.K. The analogy is that for soldiers in the kettle, it would quickly become unbearably hot.

Various other interpretations surround the term. Security expert Mal Geer told the BBC that kettling is so-called because "it takes the steam out of a potentially violent situation."

But, the BBC noted, a G20 protester in London had another view: "Kettling means keeping people inside an area until they are boiling with rage."

Where has it been used?

Kettling has been deployed by police forces in several jurisdictions, particularly in Europe, over the past 15 years.

One of the higher-profile instances took place during an anti-globalization protest in London, England, in 2001. That demonstration at Oxford Circus saw more than 1,500 people held for several hours.

Why is it controversial?

Kettling has sparked complaints of human rights violations and been criticized because law-abiding citizens and bystanders who may not be protesting have sometimes found themselves trapped behind the police lines.

Questions also surround the length of time a kettling may last, and the lack of access to water, food and toilets.

What is the Canadian experience?

The use of kettling during the G20 summit in Toronto in 2010 became very controversial.

On the evening of June 27, Ontario and city police boxed in about 400 people at Queen Street and Spadina Avenue. The incident has since been examined in various reviews and reports.

An internal Toronto police report recommended that the use of kettling — a term the report itself did not use — for containing people be modified, especially to leave an exit point when people are boxed in.

Ian McPhail, vice-chair and interim chair of the Commission for Public Complaints Against the RCMP, has said that once RCMP officers arrived at the scene in Toronto, the commander in charge raised a number of questions about the strategy.

"He was concerned about the nature of 'kettling' because that's not RCMP policy," McPhail said in an interview, noting that when it comes to crowd control, RCMP policy is to provide an exit.

A recent report by Ontario's police watchdog criticized Toronto police for using kettling during the G20, and noted that it was deployed on at least 10 occasions during the protests.

The Office of the Independent Police Review Director (OIPRD) recommended that police allow more time and use better technology to provide warnings to disperse a crowd before kettling or arresting people.

The police order to keep the group of protesters, bystanders and even some journalists boxed in at Queen Street West and Spadina Avenue "in a severe rainstorm that included thunder and lightning was unreasonable, unnecessary and unlawful," according to the OIPRD report. It violated the detainees' constitutional right against arbitrary detention and was negligent, the 276-page report says.

What have courts said?

In March 2012, the European Court of Human Rights ruled that the 2001 kettling in London was "least intrusive and most effective" tactic available to officers, the Guardian newspaper reported.

The case was the "most significant among a host of legal challenges to the crowd control technique," the Guardian reported.

The Guardian quoted the ruling: "Moreover, again on the basis of the facts found by the trial judge, the court is unable to identify a moment when the measure changed from what was, at most, a restriction on freedom of movement to a deprivation of liberty."