Tempers may have boiled up inside the G20 kettle - but police say they largely contained the violence.
By Julian Joyce
Metropolitan Police Commissioner Sir Paul Stephenson has ordered a review into whether the police tactic of "kettling", or containing, protesters is "appropriate and proportionate".
But how does kettling work? And how has it developed as the Met's tactic-of-choice when it comes to controlling large, and potentially violent, protests?
Writer and director Zia Trench thought she might visit the G20 demonstration in the City of London earlier this month.
"I wasn't sure if I wanted to protest, but I thought I'd check out the demonstration, and show my face for half an hour," she said.
"As I walked out of Cannon Street Tube I literally found myself facing a line of police. When I asked if I could get through, the police refused. They then moved me down the street towards the Bank of England."
There, Ms Trench joined a growing group of protesters corralled within a circle of police in the square in front of the Bank.
Held by the police within the circle for up to seven hours, the protesters found themselves trapped.
"We were completely surrounded - we couldn't get out and after a while the police stopped other people getting in," Ms Trench said.
Long planned by the police in the run-up to this year's G20, kettling is now the Met's tactic-of-choice when it comes to dealing with crowds of people where violence is a possibility.
As former senior policeman Andy Hayman predicted in a Times blog written the day before the summit:
"The tactics are "to herd the crowd into a pen, known as 'the kettle'... the police will not want groups splintering away from the main crowd."
Policing experts say the procedure dictates that officers move protesters to a pre-designated spot. As other protesters join the crowd, the police noose draws gradually tighter.
TERRITORIAL SUPPORT GROUP
The Met Police's Territorial Support Group (TSG) is trained to deal with possible public disorder
The 743 TSG officers were among 5,000 involved in policing the G20
TSG officers receive the highest 'level one' public order training
The TSG replaced the Special Patrol Group in 1987
Since 1987 the TSG has policed every major public order incident in London
TSG officers should be identifiable by a 'U' on their shoulder epaulette or rank insignia
TSG teams police planned events such as the G20 but also act as rapid response units
The TSG performs anti-terrorism patrols, polices football matches and provides firearms support
The TSG assists local borough officers with raids, forced entries and arresting violent suspects
A team is on call 24/7 to respond to a chemical, biological, radioactive or nuclear incident
Applicants must be recommended by a senior officer and possess 'a high degree of fitness'
Source: Metropolitan Police
Eventually protesters find themselves surrounded by police specially trained in crowd control.
Many are members of the Metropolitan Police's elite Territorial Support Group. Mounted police can also provide backup.
Police may occasionally relieve some of the pressure on the kettle by allowing people to leave - but officers are constantly seeking to maintain control and this is via a designated route.
Anyone determined to stay - like Ms Trench - may be held for hours, without access to food and water.
During the G20 events, protesters reported having to relieve themselves in the street because they had no access to toilets.
Eventually, say the police, most [protesters] get fed-up and agree to depart peacefully.
But now the tactic is under scrutiny following the death of newspaper vendor Ian Tomlinson during the G20 protest.
Investigators will also look into an alleged assault on a demonstrator the following day.
Kettling has already been unsuccessfully challenged in the courts on the grounds that it constitutes false imprisonment and that it contravenes freedom of movement.
But police defend it, arguing that the tactic provides an efficient and relatively safe way of controlling crowds who may contain individuals bent on violence.
Revealingly, the derivation of the very word differs depending on which side of the police line one stands.
Security expert Mal Geer of Prime Media, a firm instructing journalists how to cope in riot situations, says kettling is so called because "it takes the steam out of a potentially violent situation".
But a G20 protester had a different interpretation: "Kettling means keeping people inside an area until they are boiling with rage."
Liberty director Shami Chakrabarti, agrees: "We have serious concerns about "kettling" human beings in difficult situations...
"Imprisoning them in this way risks making people angry, making them desperate and exacerbating any risk to public safety," she told the BBC.
Even defenders of the tactic acknowledge it is not without risk.
"Officers find themselves incredibly up close and personal with people who are yelling abuse and trying to get out," said one former officer.
"Inevitably, tempers get frayed. You can imagine that it wouldn't take much of a spark to light the fuse."
But the Met stresses all officers are bound by strict guidelines laid down by the Association of Chief Police Officers.
For example the police standard weapon - the baton - can be used, but only if its use is "proportionate to the level of threat".
Additionally "the decision to strike is for the individual officer and must be justified by them in each instance".
It is thought the investigation into the two reported incidents of baton use, on Mr Tomlinson who later died of a heart attack on 1 April and on the protester the following day, will focus on whether guidelines on appropriate force were followed within the police kettles.
It will also consider whether kettling is used too often.
Additionally, investigators will want to know the context of the alleged assaults, and also whether the widely-circulated online video clips tell the whole story.
Police sources have told the BBC they expect the current inquiry to support the continued use of kettling.
Meanwhile, police expert Professor Peter Waddington from Wolverhampton University says the tactic owes its genesis to a dilemma faced by officers confronted with a noisy, and possibly violent, crowd. Do they disperse or contain it?
The professor remembers being unimpressed observing police dispersal tactics in action during London's violent poll tax riot in March 1990.
"There was a moment when protesters had gathered in front of the National Gallery and many had sat down. The police then decided to disperse them... and violence broke out again," he said.
"I subsequently wrote a paper arguing that it would have been much better if the police had contained the protesters by keeping them where they were."
Other highly-publicised uses of dispersal include the G8 summit in Genoa, Italy, in 2001, where police used water cannon and CS gas in an attempt to control anti-globalisation protesters.
Police kettles have already faced court challenges
One man was shot dead, hundreds of other people arrested and injured and millions of pounds of damage caused.
As for raw numbers, policing tactics at the G20 protests appears to have paid off. Despite the two instances of alleged police assault, chiefs are said to be happy there were relatively few arrests and injuries compared to previous summits.
Accused by protesters of restricting the right of protest and free movement, police kettles have already faced a challenge in the courts.
At a May Day protest in 2001, Lois Austin and 3,000 fellow demonstrators were held within a police cordon in Oxford Street for seven hours.
Although Ms Austin claimed damages for false imprisonment and unlawful detention, her case was dismissed by High Court, the court of appeal and, earlier this year, by the House of Lords.
It found her treatment did not violate her right to liberty if the measures were used in good faith, were proportionate and were enforced for no longer than was reasonably necessary.
With legal backing from Britain's highest court, combined with the belief among law and order professionals in containment as the "least worst" option, it could be that the police kettle tactic has a continuing future.
The graphic above shows how the police contain crowds of protesters, using London's Bank of England as an example. Protesters begin to gather from roads to the north and east, and are guided towards a central location by police.
A crowd of protesters forms outside the bank, as more join the demonstration from the surrounding roads. Police wearing high-visibility jackets begin to form a cordon around the crowd.
The police cordon has now entirely contained the crowd, but protesters can still enter if they so wish. Equally, people may leave the "kettle" through an exit point to the south or west, away from the main protest.
As the crowd grows and police perceive a threat, the cordon becomes a permanent "kettle" strengthened by riot police waiting in nearby vans. Nobody can enter or leave - possibly for hours.
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