MPs have warned Britain's police chiefs that they must rethink the controversial tactic of "kettling" participants of mass demonstrations.
Members of Parliament's home affairs committee said that it was not acceptable to impose a blanket ban on movement and that peaceful protesters should be allowed to leave.
The findings were contained in a report published by the committee into the G20 protests in the City of London.
More than 35,000 people took to the streets as world leaders met over two days at the beginning of April.
The police have endured a barrage of criticism in the aftermath of the death of newspaper seller Ian Tomlinson, who was hit and pushed to the ground by officers during clashes.
Independent investigators have launched several inquiries after footage of officers lashing out at protesters was caught on camera.
MPs said that images of so-called distraction tactics - shows of controlled violence against hostile individuals - shocked many and could undermine public trust.
They added that "urgent action" was required to ensure officers display identification at all times. Those who do not must face the "strongest possible" disciplinary measures, they said.
Members said that they were also concerned that untrained and inexperienced officers were left in a "highly combustible atmosphere" at the front line of the protests.
The committee added that police should not spend money on water cannons and other equipment to create distance between them and protesters. They said that the money could be better spent on training and planning, adding that police organisers must also work harder to communicate with protest organisers and the media.
Stop The War Coalition convener Lindsey German said: "Kettling is clearly unacceptable and, if it is not already illegal, it should be. The police are treating people like criminals when they have not committed any offence," she said.
Committee chairman Labour MP Keith Vaz said that the basic principle that the police must remember is that protesters are not criminals. The police doctrine must remain focused on allowing protest to happen peacefully, he said.
"In many ways, this was a large protest which passed off remarkably well. But it is clear that concerns about the policing of the G20 protests have damaged the public's confidence in the police and that is a great shame.
"The ability of the public and the media to monitor every single action of the police through CCTV, mobile phones and video equipment should mean they take even greater care to ensure that all their actions are justifiable.
"It is unacceptable for officers not to wear identification numbers at such events and any officers found to be deliberately removing their identification should face the strongest possible disciplinary measures."