Police struggle to contain demonstrators at the Big Six Energy Bash protest Jason N Parkinson/reportdigital.co.uk

Climate change activists have accused the police of using "wildly disproportionate" tactics to control a protest against an energy industry conference in the City of London.

Police said five people had been arrested for a breach of the peace and a sixth on suspicion of assaulting a police officer during the Big Six Energy Bash direct action. The protest, which aimed to draw attention to fuel poverty and "corporate control" of energy resources, took place on Thursday outside a hotel near St Paul's Cathedral in which the UK Energy Summit 2012 was taking place.

The summit, which the chief executives of EDF Energy and E.ON UK were due to attend, was billed as an event bringing together policymakers, industry leaders, investors and thinkers to discuss the UK's energy sector. The Climate Justice Collective (CJC), an activist network that organised the protest, called it "a classic 1% stitch-up", using a term coined in the Occupy movement to denote inequality of power and resources.

More than 100 protesters from the CJC and other groups, including UK Uncut and Fuel Poverty Action, gathered outside the Grange St Paul's hotel in the late morning, with many attempting to gain entry by pushing past police. Dozens were later placed inside a police kettle.

Eva Jay, an activist, said she had seen a man being hit on the head with a police baton. "I saw a police officer extend his baton and truncheon very hard on this young man's head, and then he kind of recoiled but fell down," she said. "People helped him up and took him to the side of the hotel where he slumped down and looked very shocked, very disorientated."

Riley Dylan, an activist medic, said she had treated eight people for injuries. "I saw police inflicting blows to the head and to the body," she said. "It was wildly disproportionate."

A spokesman for City of London police said he was unable to comment on the reports. Police were "containing" the protest and offering activists water, he added. "It's an ongoing incident. We're still providing a proportionate police response," he said.

Organisers of the action said they wanted to highlight their criticisms of the energy giants EDF Energy, E.ON UK, Scottish Power, British Gas, nPower and SSE, whom they attack for making large profits and for fostering the use of fossil fuels. "The UK Energy Summit is a classic 1% stitch up," said spokesman Billie Blackwood. "It is corporate elites, including the government, conspiring to keep the status quo of high energy prices, soaring profits, growing climate instability and disaster capitalism. This conference is the wrong people asking the wrong questions and proposing the wrong solutions."