A woman who monitored content on Facebook is suing the company, alleging it does not properly protect those who face mental trauma as a result of viewing distressing images.
Moderators are "bombarded" with thousands of images depicting child sexual abuse, torture, bestiality and beheadings, the legal action alleges.
It said the social network was "failing to provide a safe workplace".
Facebook said its moderators had full access to mental health resources.
The legal action has been taken in California by former contract employee Selena Scola.
She worked at Facebook's offices in Menlo Park and Mountain View for nine months from June 2017, under a contract through Pro Unlimited, a Florida-based staffing company, which is also mentioned in the legal action.
According to her lawyers, she developed post-traumatic stress disorder as a result of constant exposure to "highly toxic and extremely disturbing images" at her workplace.
The legal action says that there is potential for a class action from "thousands" of current and former moderators in California.
Facebook said in a statement: "We recognise that this work can often be difficult. That is why we take the support of our content moderators incredibly seriously, starting with their training, the benefits they receive, and ensuring that every person reviewing Facebook content is offered psychological support and wellness resources.
"Facebook employees receive these in house and we also require companies that we partner with for content review to provide resources and psychological support, including onsite counselling - available at the location where the plaintiff worked - and other wellness resources like relaxation areas at many of our larger facilities."
The social network has come under fire in recent months over how it handles fake news and hate speech on its platform and has committed to employing more content moderators.
Currently it has 7,500 reviewers, which include full-time employees and contractors.
It also uses artificial intelligence and has stated that one of its main priorities is to improve the technology so that the unpleasant job of monitoring disturbing images and videos can be done wholly by machines.