Rohingya refugees from Myanmar are suing Meta Platforms, formerly known as Facebook, for $150 billion.
The social media giant is accused of not taking action against hate speech that contributed to violence against the minority group in 2017.
In August of that year, more than 730,000 Rohingya fled Myanmar's Rakhine state after a military crackdown.
Refugees said the violence included mass killings and rape, while rights groups documented killings of civilians and burning of villages.
Myanmar authorities say they were battling an insurgency and deny carrying out systematic atrocities.
A U.S. class-action complaint filed in California on Monday argued that Facebook had failed to police anti-Rohingya hate speech.
Lawyers said that failure, and the design of the platform, contributed to real-world violence against the Muslim group.
In a coordinated action, British lawyers also submitted a letter of notice to Facebook's London office.
Facebook did not immediately respond to a Reuters request for comment about the lawsuit.
The company has previously admitted it was "too slow to prevent misinformation and hate" in Myanmar.
Whether it can be held liable for that content though is another matter.