At 4.50mins in to this clip an officer is clearly seen smacking a protestor across the head with his shield. At 7.49mins in an officer punches a protestor in the face.

EVEN amid the background shouts and screams of the G20 protests, the crack of riot shield on skull is clearly audible.

It is the moment when an unidentified riot squad officer, his face half-hidden by a black balaclava and visored helmet, was filmed using a round shield to “punch” Alex Kinnane on the left temple.

The video shows the 24-year-old IT technician from London facing away from his assailant, stationary and appearing to offer no physical threat to the police officers surrounding him. His mouth opens in pain as the shield strikes.

“I had turned around to go to someone who was screaming because they were being crushed when he reached out and hit me on my forehead with his shield,” said Kinnane last night. “I was in shock. I had to sit down and felt concussed and nauseous for over an hour. Where he hit me came up in a lump of broken skin.”

The footage of the incident, being released by The Sunday Times today, will reinforce public fears that parts of the Metropolitan police were out of control during the protests surrounding the G20 summit in London on April 1.

A second video shows another riot squad officer delivering a powerful right hook to an unidentified male demonstrator’s jaw as a crowd retreats from an advancing police line. The protester’s head jerks backwards as the punch lands.

The footage of the two incidents is part of a dossier of video and written evidence about alleged police brutality during the protests being prepared by lawyers from Bind-mans on behalf of the organisers of the Climate Camp demonstration.

In addition to the footage, the dossier contains about 400 testimonies from demonstrators claiming to have been assaulted by police or who witnessed aggressive behaviour by them.

A selection of the most serious are to be presented to MPs and the Metropolitan Police Authority, the force’s oversight body, this week.

Witness statements seen by The Sunday Times allege that police left one woman in agony after repeatedly beating her as she sat on the ground; dragged a second woman by her hair across the tarmac; and stamped on one man’s face while walking over a seated crowd.

Lily Vassiliadis, 27, whose testimony is recorded in the document, said she was sitting on the ground waiting for a friend when police charged at her. “All of a sudden they started hitting people with batons,” she said. “One officer threw me to the ground with his shield and another in a balaclava grabbed me by the hair and dragged me along the pavement. I was screaming in pain.”

Another protester, David Cullen, 29, from Manchester, said: “A whole line of policemen ran over the top of us. I had a cut lip and black eye after a policeman stamped on my face.”

The dossier will increase pressure on Sir Paul Stephenson, the Metropolitan police commissioner, who is facing questions about the behaviour of his officers during the protests. A spokesman for the Met said it was investigating the footage.

Stephenson is already under fire because of the Met’s handling of the death of Ian Tomlinson, a bystander who died minutes after being hit and shoved to the ground by a member of the Met’s territorial support group.

A second postmortem concluded last week that the 47-year-old newspaper vendor had died of severe internal bleeding, contradicting an earlier finding that the cause of death was a heart attack. The officer who struck him has been suspended. He was interviewed under caution last week on suspicion of manslaughter.

Paul King, Tomlinson’s stepson, said the shifting official accounts given of his stepfather’s death were causing his family huge stress.

“First we were told that there had been no contact with the police; then we were told that he died of a heart attack; now we know that he died from internal bleeding,” he said.

A senior Met official said Stephenson feared he was facing a “Menezes moment” — a reference to the shooting by armed police of Jean Charles de Menezes, a Brazilian mistaken for a suicide bomber in July 2005. Allegations that the Met had misled the public about the shooting by initially claiming that Menezes had acted suspiciously haunted Stephenson’s predecessor, Sir Ian Blair, until he was forced to leave his post last December.

“They are all running for cover,” the official said. “Paul Stephenson feels vulnerable . . . he fears that history might be repeating itself. Paul can see the parallels. Early in his commissionership an innocent person has died in what may be the result of police action.”

The official said that what really concerned him was that – as with the Menezes case – it had taken so long for the truth to filter up to higher ranks at the Yard.

“At the post-incident debrief, didn’t anyone think to tell their senior officer that a man had been pushed to the ground just minutes before and not far from where Tomlinson died?” said the official.

“The City of London has probably got more CCTV coverage than any other place in Britain. Yet, on hearing that a man had died, didn’t anyone seek to check the footage to see if there was anything relevant?”

Chris Huhne, the Liberal Democrats’ home affairs spokesman, said the Met seemed to be suffering from a “breakdown of discipline”. Some officers had broken police rules by failing to display their identifying numbers on their uniforms.

David Davis, the former Tory shadow home secretary, questioned the legality of the Met’s riot tactics, which involved “kettling” thousands of demonstrators into confined spaces.

A Met police sergeant was suspended last week after video footage showed him hitting a female protester, Nicola Fisher, 35, across the face and striking her legs with his baton.

The Independent Police Complaints Commission has received more than 185 complaints relating to the protest. Almost 90 are from alleged victims of or witnesses to excessive police force. The Yard has referred the cases of Tomlinson, Fisher and a third unnamed person to the IPCC for investigation.

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