A flash mob also demonstrated outside the offices for 45 minutes from midday with loudspeakers and sound systems.
The agency has been targeted for its work with client EON, which owns the controversial Kingsnorth power station, the planned site of a new coal-fire power station.
The Climate Camp activists said they wanted to ‘expose the naked truth behind Edelman’. Activist Alice Fielding said: ‘Edelman are nothing more than new coal spin doctors, intent on making profit out of EON’s activities at the expense of the global climate.’
Edelman CEO Robert Phillips told PRWeek that the agency had increased security at the offices because they suspected they would be targeted.
'The protesters didn't seem very organised. We offered them a conversation, but that seemed to confuse them. They are engaged in old-fashioned PR, they just wanted to make noise rather than get their point across through conversation', he said.
He added: 'But apart from the flash mob, the protest was peaceful and rather civilised.'
Phillips used Twitter to update journalists on developments, as well as offering the protesters ‘coffee and a (clothed) conversation about their issues’.
‘Let us know if you decide to talk, rather than sing,’ said a recent tweet.
Edelman was targeted in July 2008 by activists from Oxford Climate Action, for its relationship with EON.
Climate Camp 2009’s other targets include The Royal Bank of Scotland, the headquarters of BP and Shell, and Heathrow Airport were among the other offices being targeted.