Senior Met chiefs should face possible disciplinary action over the failure of their officers to display their identification numbers during the G20 protests, the chairman of a powerful Parliamentary Committee said today.

Andrew Dismore, who heads Parliament's Joint Committee on Human Rights, said: "The buck should stop," with those in charge of policing the demonstration and that action should not be restricted only to rank and file officers.

He even suggested that sanctions could extend to Met Commissioner Sir Paul Stephenson, saying that responsibility might extend to: "The man at the top," although he declined to name any specific officer whose failure had led to officers policing the event without identification.Today's call by Mr Dismore, the Labour MP for Hendon, came as the committee heard evidence from the Independent Police Complaints Commission, Police Chief and demonstrators about the April 1 protests.

At the hearing, the IPCC's Chairman Nick Hardwick confirmed that a previous investigation by his organisation into the policing of the Countryside Alliance protests of 2004 had already raised concerns, which the Met had promised to act upon, about officers failing to display their identification numbers.

He said that such practices were unacceptable and that the new investigation into the G20 protests would assess whether the renewed failure of Met officers to wear their serial numbers should lead to disciplinary action.

Mr Dismore welcomed his commitment but urged the IPCC to consider acting against senior Met officers, as well as rank and file police if it emerged that supervision and enforcement of police orders had been deficient.

He added: "If an order was given and it isn't followed then where does the buck stop? Surely it is the man at the top," he told the Committee.Met Commissioner Sir Paul Stephenson and the Governments Chief Inspector of Constabulary Denis O'Connor had both already stated that the failure of some officers to display their identification numbers during the G20 demonstrations was unacceptable.

The IPCC is currently investigating the issue after receiving a total of 262 complaints relating to all aspects of the policing of the G20 protests. Although 140 of these are outside the organisation's remit, the other 122, which relate largely to allegations of assault, are being probed.

Meanwhile, a climate change protester today told how he was "beaten up" without warning by police at the G20 demonstrations in London.

In evidence to the Commons Home Affairs Select Committee, Chris Abbott, a 30-year-old academic, said he was "punched in the face" and then "smashed" across the head with a riot shield as he sat on Bishopsgate during the 1 April protests.

He told MPs that the alleged assault was carried out by four or five officers wearing balaclavas and came even though he was sitting peacefully with his arms raised above his head offering no resistance.

Mr Abbott's testimony came as MPs on the committee conducted the latest in a series of hearings to investigate the Met's handling of the G20 protests and the future of public order policing.

An independent police complaint's commission investigation is already underway into police conduct during the demonstrations - at which news-vendor Ian Tomlinson died after clashing with officers - amid widespread allegations of alleged police misconduct and excessive use of force.

Such concerns will be heightened by Mr Abbott's evidence today in which he described how he had arrived at around 6pm to join the climate camp protest in Bishopsgate.

Mr Abbott, the deputy director of the London based Oxford research think tank, which analyses international security and foreign affairs, told MPs that by 7pm a police "kettling" cordon had been set up around the camp but that the atmosphere remained friendly and relaxed for a further two hours.

The situation suddenly changed, however, when a line of officers suddenly arrived and began to clear protesters without warning and assaulting Mr Abbott in the process.

He told MPs: "An officer leant over and punched me squarely in the face for no valid reason. Then they descended on me with their shields striking me on the side of the head. Whilst they were attacking me I had my hands in the air and offered no resistance whatsoever."I was assaulted by police."

Mr Abbott said that his girlfriend had also been accosted by officers and accused the police of ignoring the relaxed atmosphere of the protest and denying environmentalist their legitimate right to demonstrate.

"I didn't expect to be beaten up by police when you are exercising your right to demonstrate over an issue as important as climate change."

In other evidence, Nicola Fisher, from Brighton, also accused the police of assaulting her the following day as she attended a vigil at Bank Station for Mr Tomlinson.

She said of the alleged attack - footage of which has been widely broadcast on television - involved an officer slapping her across the face and then hitting her with a baton across the leg on two occasions.

She admitted, however that immediately before the incident she had pushed the officer concerned, after first being pushed by him, and had also sworn angrily, although she insisted that the officer's action remained unjustifiable.

Asked by MPs about reports that she had received 50,000 for selling her story to the media, Ms Fisher said the figure was "ridiculous" but declined to say how much she had received.

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