• Climate Camp participants struck with shield and punched • Third complaint against Met in wake of Tomlinson death

The policing of the G20 protests in London has come under further scrutiny with the release of a video that appears to show at least two instances of unprovoked violence by riot officers against demonstrators.

The Independent Police Complaints Commission (IPCC) said last night it was investigating a third instance of potential police misconduct during the demonstrations on 1-2 April after a complaint from a 23-year-old man who says he was assaulted at a police cordon near Cornhill in the City.

Two officers from the Metropolitan police territorial support group (TSG), which deals with public disorder situations, have already been suspended over their behaviour during the demonstrations.

The new footage, which is not related to the cases being investigated by the IPCC, has been compiled and edited by activists. It concerns incidents during the evening of 1 April when riot police broke up the so-called Climate Camp, an action by anti-global warming activists on Bishopsgate in the City of London that had to that point been peaceful.

Firstly, in the excerpt shown above, as officers attempt to push the tightly packed crowd of activists back on the street, one officer can be seen striking the front of his riot shield with apparent force against the head of one man, who is not resisting and has just turned his head away from the police. The riot officers involved have their faces covered with balaclavas and some appear not to be showing police numbers on their uniforms.

The protester involved has been named as Alex Kinnane, a 24-year-old IT worker.

Later that evening, in the excerpt below, as police try and push back another crowd at the camp a riot officer appears to punch a demonstrator in the face with a clenched fist.

The Guardian has already received testimony from a series of people involved with the climate demonstration claiming that police used excessive and unjustified force in breaking it up. Other video footage from the camp shows police using their batons and the edges of their riot shields to strike protesters who appear not to be resisting.

Neither of the latest incidents, or any other at the camp, is thus far the subject of a formal IPCC investigation.

The Guardian has additionally passed to the IPCC a series of further videos showing apparent excessive force or questionable tactics by police at the G20 protests.

One TSG officer faces investigation in connection with potential manslaughter charges after video footage was published by the Guardian showing Ian Tomlinson, a 47-year-old newspaper vendor, being struck on the legs and pushed to the ground as he tried to get home. Tomlinson collapsed and died shortly afterwards. While an initial postmortem blamed heart failure, a second examination concluded that he died from internal bleeding.

Another case involving a TSG officer is being looked into in connection with video footage showing him slapping and using a baton on Nicola Fisher, 35, from Brighton, East Sussex.

• This article was amended on Monday 20 April 2009. In the sixth paragraph, we originally misspelt Alex Kinnane's name as 'Cinnane'. This has been corrected.