Activists from Oxford Climate Action arrived at the London office of Edelman on Wednesday morning, alleging a massive 'greenwashing' campaign by the agency's energy client E.ON.
Several protesters gained access to the PR firm, while others climbed on to the roof to unfurl a banner.
Labour MP John McDonnell stopped by to lend his support to Oxford Climate Action's stunt.
It said the demonstration aimed to 'highlight the insanity of investing in new coal power stations such as Kingsnorthin the face of human-caused climate change'.
The group also described Edelman as having 'a long record of involvement with ethically questionable companies, including AstraZeneca, Wal-Mart and Pfizer'.
Click HERE to watch Edelman CEO Robert Phillips talking about the protests.
Protester Debbie Locke said: 'Edelman is telling E.ON to say that Carbon Capture and Storage [CCS] can help, but CCS is an unproven technology and 20 years away at the earliest. We are here to reclaim the PR machine for normal people who want to see real action on climate change, not another dose of corporate greenwash.'
Edelman's UK CEO Robert Phillips posted a blog entry and video after the protest, but expressed disappointment that the campaigners did not want to speak to him. 'I would be absolutely happy to have a free and open debate with them, but they aren't interested,' he said. 'We are proud of ukour work with E.ON in tackling climate change issues.'
The PRCA hit out at the protesters. Director general Francis Ingham said: 'These people should stop childishly ranting against things they disagree with, and start talking.'