G20 protesters have complained about police tactics in London
Britain may have entered a new era of overt violent policing that could deter people from protesting in future, the Green Party's Jenny Jones has said.
She says protesters' stories of police brutality at the G20 demonstrations and the police's story of complete professionalism "just don't stack up".
Ms Jones, a Metropolitan Police Authority member, has written to the Met Commissioner about her concerns.
The Met have said their tactics were "proportionate and worked".
Police made 122 arrests during the protests in London on Wednesday and Thursday.
Some 5,000 people took part in the demonstrations that were mainly peaceful and good-humoured but marked by sporadic violence.
Ms Jones said she was particularly concerned about the treatment of protesters outside the Bank of England, many of whom were kept inside a police cordon for several hours.
They were trying to agitate and hijack that protest - we believe our tactics were proportionate and worked
"If the police were hitting people who were sitting down with their hands up, we have entered a new era of overt violent policing in Britain that will deter people from protesting and cut back our civil liberties," she said.
She went on to say that people at the Climate Camp in Bishopsgate reported that they were given no warning they were about to be contained by police.
The Metropolitan Police have defended the strategies used in Operation Glencoe, the name given to the the police plan.
Speaking on Thursday, Commander Simon O'Brien said only a small proportion of the protesters outside the Bank of England took part in the violence.
"They were trying to agitate and hijack that protest. We believe our tactics were proportionate and worked. The lawful protesters were the victims of them, not the Metropolitan Police," he said.
The police estimate the number of hardcore trouble-makers stands at between 300 and 500.
Responding directly to Ms Jones' comments, a Met Police spokesman said there would be a "post-event evaluation" to look into the police's response to incidents.
Ms Jones said she had been assured that Metropolitan Police Authority members would be able to ask questions about the police operation at its next meeting on 13 April.