Senior police officers face serious questions over the "unacceptable" trend of officers disguising their identity during clashes with protesters, the chair of the independent police watchdog warned yesterday, as it began formally investigating a third alleged assault on a G20 protester.

Nick Hardwick, chair of the Independent Police Complaints Commission (IPCC), called for a national debate over how police maintain public order and demanded much tougher political accountability, warning that police should remember they were "the servants not the masters" of the people.

He is also seeking the necessary resources for the watchdog to conduct more investigations independently from police - as it is doing over the death of Ian Tomlinson, the news vendor who died after being caught up in the G20 protests - and expanding its remit in cases where there is evidence of wider systematic problems.

The latest investigation concerns a 23-year-old man who claims to have been assaulted by a Metropolitan police officer in the early evening of 1 April at a police cordon on Cornhill in the City of London, adding to two existing investigations into the death of Tomlinson and claims by a woman activist that she was attacked.

Hardwick told the Observer the latest case would "not necessarily" be the last taken up by the IPCC, which is still sifting almost 90 complaints about the use of force and examining CCTV footage.

He made clear his concerns about incidences of officers disguising their identifying numbers, which should always be displayed on the shoulders of their uniforms, arguing that colleagues should have reported such wrongdoing.

"I think that raises serious concerns about the frontline supervision," Hardwick said. "Why was that happening, why did the supervisor not stop them? What does that say about what your state of mind is? You were expecting trouble?

"I think that is unacceptable. It is about being servants, not masters: the police are there as public servants."

He said such infringements were within the IPCC's remit "and we will deal with it".

Hardwick also revealed that the widespread use of mobile phones by protesters to take photographs and video footage of the clashes was providing invaluable evidence.

He suggested that had such footage been available during a violent confrontation between police and Countryside Alliance activists protesting over the hunting ban five years ago the outcome might have been different.

"What's been important with all these pictures is we have got such a wide picture of what happened," he said. "I think that is challenging the police: they have to respond to the fact that they are going to be watched, there is going to be this evidence of what they have done."

Hardwick said that while the IPCC had attempted a number of prosecutions over the Countryside Alliance demonstration these had failed: "We had to go with what the court said but we were very very surprised at some of the verdicts. I don't think this would happen now because there would be all this evidence."

He welcomed an inquiry announced last week by HM Inspectorate of Constabulary into police tactics during the G20 protests but said a broader debate was needed, particularly about the rights of peaceful protesters in marches liable to be disrupted by a small criminal element. "It's got to be a democratic political question about how do we want to be policed? I think that needs a proper parliamentary discussion. The choices we make as a society about that aren't consequence-free. There are tricky balances to be struck."

The IPCC has been widely criticised for its initial handling of the G20 complaints, but Hardwick said its remit was restricted to the conduct of individual officers and some of the criticism reflected misunderstandings of its role.

He is now pushing for extra manpower to conduct more investigations - a third of the IPCC's investigators are working on the G20 cases - and a shake-up of police accountability whether through elected police boards, commissioners or mayors to look at what he said were "perfectly legitimate, proper questions" outside the IPCC's remit.

Chris Huhne, the Liberal Democrat home affairs spokesman, said his party was given a guarantee by senior officers after clashes with climate change protesters at Kingsnorth power station in Kent - where there were also complaints of police disguising their identity - that there would be no repeat of such tactics at the G20 protests.

Some officers now appeared prepared to flout recent orders from senior commanders to display their numbers, Huhne said, with another officer photographed at the protest staged by Tamils in Parliament Square with his numbers disguised. "What we appear to have is repeated cases of police officers ignoring the direct orders of their police supervisors and this is very worrying.

"There's only one motive for a police officer disguising his identity and that's because he thinks he's going to be doing something reprehensible."

Senior Metropolitan police officers held a series of crisis meetings throughout last week and sources said Sir Paul Stephenson, the new commissioner, was determined to get a grip. One Met source said he was ready to "kick some ass" among senior officers. The IPCC has received more than 185 complaints about the G20 protests, of which 44 are not eligible for consideration, including complaints from people who saw footage on TV. Around 90 complaints about use of force included witness accounts as well as those from alleged victims.