The protesters, who represent anti-fracking group Reclaim the Power, threw leaves, spread manure and sprayed 'squid ink' on the windows. Reclaim the Power said 40 protesters took part.
A spokeswoman for Bell Pottinger said: "The protesters left before we even realised they were in the building. They did not enter Bell Pottinger's offices and there was no disruption."
Activist Ash Hewitson said companies like Bell Pottinger were "crucial for the fracking industry". He said: "Through their partnership with Cuadrilla and Centrica, they work to further the fracking agenda and suppress the stories of resistance and opposition on the ground."
Fracking - or hydraulic fracturing - is a controversial practice that involves pumping a combination of water and chemicals into a drilled well. The mixture then fractures the shale rock beneath the earth's surface and allows trapped gas to escape.
Last week the group staged a similar demonstration, in which three women dressed as brides chained themselves to the door of the agency St Brides Partners.
In that instance protester Hahhan Dow said the group was calling on St Brides "to divorce companies' intent on ignoring public concerns around unconventional oil and gas extraction".
Today's demonstration concludes a fortnight of action in which Reclaim the Power targeted companies it considers to be within the fracking industry supply chain.