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Screenshots from various Twitter users appeared to show several anonymous accounts sharing accusations of sexual abuse against Apa, Cole Sprouse, Lillie Reinhart and Vanessa Morgan.
The since-deleted false accusations made against the stars ranged from rape to groping and sexual harassment.
"Do you see how easy it is to lie and you guys will believe it? Vanessa Morgan and KJ Apa didn't do jack shit. You will believe anything."
After the tweets went viral, both Sprouse and Reinhart denied the sexual assault accusations.
"Earlier today myself and three other castmates were falsely accused of sexual offence by anonymous accounts on Twitter," Spouse wrote. "I take these accusations very seriously, and we will be working with the right teams to get to the root of it."
"False accusations do tremendous damage to victims of actual assault," the actor added. "Furthermore, I would never seek to silence anybody. I encourage that people look into the accusations themselves, as the events detailed were factually untrue."
"This seems to be the latest claim in a series of incidents seeking to baselessly cancel my castmates and me," he added.
Meanwhile, Reinhart wrote: "I have always taken sexual assault allegations seriously. But it was proven that this account was created specifically to create false stories about me and my cast."
"I can't think of something more twisted than lying about sexual assault," Reinhart continued. "It invalidates the men and women who are brave enough to come forward with the truth. This kind of lie can ruin lives and careers – and I can call it a lie because the person who made the allegations already admitted that the stories were fabricated."
Reinhart added that she is taking "legal action" and said, "this kind of sick behaviour only harms true survivors."
Riverdale actress Camila Mendes also came out to defend her co-stars over the allegation on her Instagram story.
"It's incredibly destructive to falsely accuse people of sexual assault, whatever point this person was trying to make about how quickly people believe what they read, it was not worth damaging the integrity of the me too movement," Mendes wrote.
"It's sickening and doesn't prove anything except that there are twist people in this world who have no concern for the consequences of their harmful actions.
On the next slide, Mendes added, "Sending love to my castmates who had to deal with this nonsense. Love you guys."
Apa has not made any public comments about the false sexual assault accusations.