Police officers must remember they are "the servants and not the masters" of the people, the head of the police watchdog said yesterday as he condemned the practice of concealing shoulder numbers as "unacceptable".

Nick Hardwick, the chairman of the Independent Police Complaints Commission (IPCC), said he would appear before the Home Affairs Select Committee as the fallout surrounding allegations into the use of police force at the G20 protests grew more acrimonious.

Mr Hardwick said he had "serious concerns" about front-line supervision of officers at this month's demonstrations. The IPCC launched a third investigation yesterday into police tactics after a 23-year-old London man claimed he was assaulted by a Metropolitan Police officer. "Why was that happening? Why did the supervisor not stop them? What does that say about what your state of mind is?" he asked. "You were expecting trouble?"

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Mr Hardwick's comments came as former Metropolitan Police Commander John O'Connor warned that the current Met chief, Sir Paul Stephenson, risks losing the support of rank and file officers if the investigation into allegations of abuse became a "witch hunt."

And Sir Ken Jones, the president of the Association of Chief Police Officers (ACPO), moved to defuse the situation by praising the "vast and overwhelming majority" of officers who, he said, deserved public support.

The chairman of the Home Affairs Select Committee, Keith Vaz, said MPs would examine the G20 controversial "kettling" tactic used to contain demonstrators as well as "what happened at the G20 protests" and the scope of the IPCC investigation.

A new video emerged yesterday of two further incidents which occurred at the Climate Camp demonstration in Bishopsgate. The first involved Alex Cinnane, 24, an IT technician from London, who is shown being hit on the head with a riot shield by an unidentified, partially masked officer. The other shows an officer punching an unidentified male demonstrator in the jaw as the crowds retreat from an advancing police line.

By Friday, the IPCC had received more than 185 complaints about the G20 protests, including 90 about the use of force, from witnesses as well as alleged victims.

Mr Hardwick said that the recent trend of digital cameras and mobile phones for photographs and video footage was providing the IPCC with invaluable evidence as well as "challenging" police.

"They have to respond to the fact that they are going to be watched, there is going to be evidence of what they have done," he said. Mr Vaz said that the amount of amateur and CCTV footage meant police actions were "much more open and much more transparent".

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Mr Vaz also called for a broader debate about the right to protest and the responsibilities of policing. "It's got to be a democratic question of how do we want to be policed? That needs a proper parliamentary discussion," he said. "The choices we make as a society about that aren't consequence-free. There are tricky balances to be struck."

But temperatures were rising among protesters as some warned that this year's May Day demonstrations could be some of the most dramatic yet. The United Campaign Against Police Violence, a coalition formed in the wake of the G20 protests, met last weekend to discuss plans. "We're trying to stop London on May Day in memory of all those who have died in police custody," said Chris Knight, a founding member. "If police officers haven't got the message at that stage, we may have to take it into our own hands."

Others have called for riot police to wear large football-shirt style numbers on their fronts and backs so that it is harder for them to conceal their identity. A report published by The Camp for Climate Action over the weekend deemed policing as "violent and disproportionate" though not dissimilar to the type of policing experienced at other climate camps.

Incident log: The Met under fire

*1 April 2009

Ian Tomlinson, a newspaper vendor, is struck with a baton and pushed to the ground by a police officer. He is seen remonstrating with the police who offer no assistance leaving it to bystanders to help him back to his feet. Later he dies. Officer is suspended.

*1 April 2009

An unnamed 23-year old man claims he was assaulted by a police cordon in Cornhill. The case, which has not yet come to light on video, is the third under investigation by the IPCC.

n 2 April 2009

Nicole Fisher, from Brighton, is seen remonstrating with a police officer, who slaps her with the across the face before drawing a baton and then striking her on the legs. The officer, whose shoulder number is concealed, is identified and suspended.

*1 April 2009

Alex Cinnane, 24, an IT technician from London, is hit on the head with a riot shield by an unidentified officer whose face is half-hidden by a balaclava. Another video, pictured far right, taken the same day shows an officer delivering a right hook to an unidentified male demonstrator's jaw as the crowds retreat from an advancing police line. Yesterday, a spokesman for the IPCC said that no formal inquiry into these cases had yet begun.