Climate Camp pitched tents in the streets
Solicitors representing G20 protesters are warning the Metropolitan Police they may seek a judicial review of the tactics used at the summit.
Members of the Climate Camp group want the Met commander to explain why force was used and why crowds were "kettled" - or contained - for several hours.
They have sent a "pre-action" letter to Sir Paul Stephenson demanding answers.
The Independent Police Complaints Commission is currently investigating the policing of the G20 protests.
Among the cases the IPCC is investigating is the death of newspaper seller Ian Tomlinson.
He died shortly after he was seen being pushed over by a police officer during the protests on 1 April in the City of London.
The letter, sent by law firm Bindmans, warns Sir Paul he may face a judicial review unless he explains the decision to authorise the use of batons and shields by officers.
Among the other "unlawful decisions and actions" it claims were committed by the Met, the letter also demands that senior officers provide a legal basis for the use of kettling.
Solicitor John Halford said justification for kettling remained "entirely opaque" and "no reason" had been given for keeping the kettle in place for more than four hours.
"The spontaneous and unprovoked use of batons by riot gear-wearing police... is not mentioned much less explained" in the Metropolitan Police Authority's report into the event, he continued.
Climate Camp says it has set up a "fighting fund" with a target of £40,000 to pay for any legal action. However, the group says it is likely that many of those bringing cases will be eligible for legal aid.
Protester Hugh Morris said: "The policing of Climate Camp in the City was totally disproportionate and in all probability, unlawful.
"Yet again we are faced with another David and Goliath battle in the courts to put this right when the real battle should be in halting climate change.
"After all, that's why we were in Bishopsgate."
The IPCC is currently investigating the circumstances surrounding Mr Tomlinson's death. It is also looking into three other allegations of violence by police officers at G20.
Sir Paul has already called in HM Chief Inspector of Constabulary Denis O'Connor to conduct a review of public order tactics.
Mr O'Connor will be questioned by members of the Home Affairs Select Committee this week on the actions of officers.