The protestors, who represent anti-fracking group Reclaim the Power, said PR firms that are "wedded to the fracking industry are locking the UK into a toxic marriage that funds dangerous and unwanted unconventional oil and gas in the UK".
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.
Protestor Hannah Dow, who locked herself by the neck, said: "We're gathered here today to witness a company representing projects for the richer, not the poorer, and we're calling on St Brides to divorce companies' intent on ignoring public concerns around unconventional oil and gas extraction."
According to the group, St Brides works with a number of oil and gas firms, including Europa Oil & Gas and Cluff Natural Resources. The agency was yet to comment at the time of publication.
Today's protest forms part of a fortnight of action in which Reclaim the Power plans to target companies it considers to be within the fracking industry supply chain. The group told PRWeek it would be "hard" to give information on which other PR firms may be targeted or when.
According to its website, St Brides is an investor and financial public relations and marketing consultancy for growth companies, and was founded in 2001.
In a similar incident in 2015, demonstrators targeted the offices of Edelman and Media Zoo, which represent companies involved in fracking.
Last year, the group who entered Media Zoo's offices pleaded guilty to aggravated trespass at Hammersmith Magistrates Court and received a conditional discharge.