Her Majesty's Inspectorate of Constabularies, the police watchdog, has published a highly critical report of how forces are managing protests and public order situations in England, Wales and Northern Ireland. There are different approaches for different circumstances. So what's the result in different situations?
Camp for Climate Action: Drax power station, September and August 2006
A large number of climate change activists converged on the Drax power station in Selby, North Yorkshire.
Officers were deployed into the camp
One of the camp's billed events was the "Battle for Drax", which Her Majesty's Inspectorate of Constabularies (HMIC) said had been more of a festival than anything sinister.
But there were arrests at the demonstration, including those of people carrying bolt-cutters.
The force's own review said there had been a "community" aspect to the protests and it had been a good idea to assign "neighbourhood" roles to some officers.
They were deployed into the camp to maintain a dialogue with the protesters.
North Yorkshire's review did however question some of the force's tactics, including whether it had had enough adequately trained officers.
Kingsnorth Climate Camp: July and August 2008
This was a similar event in Kent, involving 2,000 protesters. It has now become a cause celebre for campaigners who say the police powers need to be curtailed.
Some MPs said police tactics had been completely over-the-top
The protesters took over private land and issued statements saying they were going to close down the Kingsnorth power station.
Some 1,400 officers from 26 forces came to the aid of Kent Police in what became a massive operation designed to prevent anyone illegally entering the complex.
Police were soon accused of heavy-handed tactics, including carrying out 8,218 stop-and-searches, many without any clear reason.
Some 600 items were seized from protesters, and that in turn contributed to 146 complaints.
Police arrested 100 people and charged 46 of them.
In the aftermath, some MPs said the police tactics had been completely over-the-top.
The National Police Improve Agency's review concluded there had been no proper assessment of the threats posed by the protesters.
In many cases, officers were not told what they were searching for - and commanders did not keep a dialogue going with the camp.
G20 demonstrations: London, April 2009
The G20 protests in London saw thousands of protesters take to the streets during the two-day world summit.
There was no single demonstration - but a great deal of expectation the capital could grind to a halt because of all the different actions.
Scotland Yard said its G20 policing plan had been challenging
The Metropolitan Police and its supporting forces assigned 84,000 officer-hours to the operation and warned there would be a "swift and efficient policing response" at the first sign of trouble.
Large teams of officers in full riot gear took control of parts of the City of London when they were met by a small number of trouble-makers.
But their strategy to "kettle" the protesters by the Bank of England sparked furious complaints from people who said they had been held against their will.
Demonstrators used mobile phones to record the police's actions - this footage is now playing a significant role in the aftermath, not least that relating to the death of Ian Tomlinson, who was struck and pushed by an officer.
The separate Climate Camp in the City, an occupation of a major road, was entirely peaceful - but highly disruptive for anyone in the area.
Officers were told to break it up late at night, leading to complaints they had gone in hard rather than reaching a deal with the demonstrators.
The HMIC report into the G20 criticised the Met for being inadequately prepared for the complex and different protests they would meet.
Civil liberties campaigners say officers were given a one-size-fits-all strategy of confrontation and containment rather than facilitation where appropriate.
Tamil protests: Parliament Square, April to June 2009
This was the other massive protest in London around the same time as the G20.
British Tamils lobbied the government to intervene in the conflict in Sri Lanka and occupied Parliament Square.
The protests were allowed to continue and slowly wound down
The protest swelled to as many as 100,000 people and lasted 73 days.
Two demonstrators jumped off Westminster Bridge and others went on hunger strike.
This was a huge challenge for the police because nobody was leading the protests and officers struggled to maintain a dialogue.
Tamil police officers were sent into the crowds to try to calm things down. At one point, the Met used Bluetooth technology to send texts to mobile phones, setting out the police's position.
The protests were allowed to continue and slowly wound down.
English Defence League: 2009
The English Defence League, an anti-Islamist organisation, has been causing the police significant concern with its loosely structured protests.
There were a number of scuffles in Birmingham city centre
Each demonstration sees a response from anti-racism groups and, in some cases, local Muslims who feel they need to "defend" themselves.
In August 2009, police took the unusual step of seeking to ban a march in Luton, saying they could not guarantee public safety.
The HMIC has now called on the Home Office to provide clearer guidance to forces on when a march can be banned.
The following month, the English Defence League marched in Birmingham. There were a number of scuffles in the city centre.
The HMIC says the police operation successfully kept the different groups separate, avoiding shutting down parts of the city centre.