Most police officers receive two days of public order training every year
Too many inexperienced police were on the front line of London's G20 protests, according to a report by MPs.
The Commons home affairs committee said confidence in police had been damaged because of some officers' actions, despite the operation's successes.
One man's death is being probed and there have been a series of complaints.
But the Metropolitan Police said it was "wrong" to suggest officers were not trained and doing so would "damage public confidence".
The committee also criticised the police's widespread use of crowd containment - a controversial strategy known as kettling.
Some 35,000 people are thought to have taken part in the numerous protests in London during the world leaders' summit in April.
Officers had to work 10,000 shifts across the capital.
The Independent Police Complaints Commission is investigating the death of Ian Tomlinson, who died minutes after being pushed over by a police officer.
Dominic Casciani, BBC home affairs correspondent
When prospective constables enter training college, they're told that British bobbies uniquely "police by consent" of the public.
Many G20 protesters say that consent was abused and the home affairs committee report acknowledges that grievance - although it adds some demonstrators didn't exactly help.
More reviews will follow: The chief inspector of constabulary is looking at public order tactics - and the IPCC watchdog is investigating specifics, including Ian Tomlinson's death.
Some demonstrators want to legally challenge "kettling" - although judges have previously supported the tactic.
Before the G20, the Met said it would deal robustly with violence; the question being asked is whether officers should have talked more to protesters beforehand.
Whatever happens, camera phones and the internet are being used to scrutinise police like never before. Officers will inevitably face more calls to show they've remembered that first principle from training.
It is also looking at three other complaints of violence on demonstrators.
One of the cases involves a woman, Nichola Fisher, who was slapped in the face by an officer.
In their report, the MPs from the committee said that despite little time for planning and limited information from the protesters themselves, the policing strategy had been largely successful.
There had been very little disruption for London and only minor acts of violence, they said.
But the MPs said the success was partly down to luck on the day, particularly because too many untrained officers had been on the front line of protests.
Most police officers receive two days of public order training every year, although specialist officers train almost once a month.
"Never again must untrained officers be placed in the front line of public protests," said the report.
"At the very least each unit should contain a core of fully trained, experienced officers. While greater funding must be made available, the police must also allocate their resources better to ensure that all officers on the front line of public protest are trained adequately."
The committee also called for kettling to be used sparingly.
"The police must have reasonable grounds to believe that the protesters being contained are liable to cause disturbances elsewhere and innocent bystanders and non-violent protesters must be allowed to filter out," it said.
"It is not acceptable for a blanket ban on movement to be imposed.
Nichola Fisher describes being hit in the face - Archive interview, April 2009
"The police must be aware that their behaviour will be monitored, recorded and instantly made public via the internet. They must modify their behaviour and briefings accordingly."
Chairman of the committee Keith Vaz said the public "clearly don't understand" the reasons for using kettling and other public order strategies.
"What's acceptable, what's within the police rule book - the use of distraction tactics, for example, slapping or hitting people - shocked the public," he told the BBC.
"Therefore, they need to look again at these tactics and consult the public to decide whether or not such tactics can be used again."
Mr Vaz also said relations between police, protesters and the media must be better in the future.
"We felt it was important that the police understood there is a right to protest," he said.
"Those who go out on the streets are not criminals... what the police should be focused on is making sure the protests are allowed to happen peacefully."
Some of the protesters who believe they were unfairly detained during G20 are set to launch a legal action against the Met.
The Met's Assistant Commissioner Chris Allison said the force would learn lessons from G20.
"Only by doing this will we maintain the confidence of Londoners," he said.
Witnesses say Mr Tomlinson collapsed after earlier contact with police
"However, it is wrong to suggest that our officers are not trained. They are. To suggest otherwise can only serve to damage public confidence in us.
"G20 was a complex and hugely demanding operation managing the protection of heads of states, while balancing the right to lawful protest and maintaining public order on our streets.
"It should not be forgotten what was achieved and this report does highlight that it was a remarkably successful policing operation."
Chief Constable Meredydd Hughes, of the Association of Chief Police Officers, said: "Policing must be proportionate, lawful, accountable and necessary, and we accept that we will not get it right every time.
"What's important is that we take those opportunities, encourage debate and as a service, respond appropriately to criticism."
And Paul McKeever, chairman of the Police Federation of England and Wales, said officers accepted that kettling led to some people being detained when they were not involved in a protest.
"What has to be considered is the alternative," he said. "By choosing a method such as dispersal and allowing groups of demonstrators to roam around areas at will, you are allowing, at best, disruption to the lives of non-protesters and at worst serious disorder."
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