Matt Bergman, founding attorney of Social Media Victims Law Center, said the Meta CEO`s testimony is "a moment that families across this country have been waiting for."

Meta CEO Mark Zuckerberg arrives for a landmark trial over whether social media platforms deliberately addict and harm children, Wednesday, in Los Angeles.Ryan Sun / AP

Meta CEO Mark Zuckerberg is testifying in a landmark trial that could set a precedent for whether social media platforms are responsible for harming children.

It`s the first of a consolidated group of cases - from more than 1,600 plaintiffs, including over 350 families and over 250 school districts - scheduled to be argued before a jury in Los Angeles County Superior Court. Plaintiffs accuse the owners of Instagram, YouTube, TikTok and Snap of knowingly designing addictive products harmful to young users` mental health.

Historically, social media platforms have been largely shielded by Section 230, a provision added to the Communications Act of 1934, that says internet companies are not liable for content users post. TikTok and Snap reached settlements with the first plaintiff, a 20-year-old woman identified in court as K.G.M., ahead of the trial. The companies remain defendants in a series of similar lawsuits expected to go to trial this year.

K.G.M., who was a minor at the time of the incidents outlined in her lawsuit, claims that her early use of social media led to addiction and worsened her mental health problems. Her lawsuit alleges that social media companies made deliberate design choices to make their platforms more addictive to children for purposes of profit.

Her attorney, Mark Lanier, appeared optimistic when arriving at court, telling reporters it`s "going to be a good day."

Zuckerberg`s testimony comes one week after Adam Mosseri, the head of Meta`s Instagram, defended the platform in court, arguing that social media platforms are not intentionally engineered to be addictive.

"I think it`s important to differentiate between clinical addiction and problematic use," he said when he was pressed about social media addiction.

Matt Bergman, founding attorney of Social Media Victims Law Center - which is representing about 750 plaintiffs in the California proceeding and about 500 in the federal proceeding -called Wednesday`s testimony "more than a legal milestone - it is a moment that families across this country have been waiting for."

"For the first time, a Meta CEO will have to sit before a jury, under oath, and explain why the company released a product its own safety teams warned were addictive and harmful to children," Bergman said in a statement Tuesday, adding that the moment "carries profound weight" for parents "who have spent years fighting to be heard."

"They deserve the truth about what company executives knew," he said. "And they deserve accountability from the people who chose growth and engagement over the safety of their children."

Outside the courtroom last week, parents held up photos of their children as they waited for Mosseri to emerge from the proceedings.

Zuckerberg previously apologized to some parents when testifying before Senate last year during a hearing regarding the online child safety.