3:52 PM PT -- 50's attorneys, Reena Jain and Blank Rome, tell TMZ … "Despite being given ample opportunity to retract a false and malicious retaliatory accusation, Ms. Narvaez has shamefully chosen to interfere with her 11-year-old son’s relationship with his loving father by falsely calling him a 'rapist.'"

They add, "The motivation behind this appears to be Ms. Narvaez’s unfortunate entanglement and misguided loyalty to Mr. Combs, who we believe to be underwriting this attack and whom Mr. Jackson has been warning Ms. Narvaez and others about for many years."

50 Cent's through trolling his ex, now he's brought lawyers into it, and he's suing Daphne Joy for publicly accusing him of rape and physical abuse.

In docs filed Monday, and obtained by TMZ, 50 blasts Daphne for her Instagram post from March 28, where she made the allegation behind his lawsuit -- and he says she only did it because it came out that he was going to seek sole custody of their son, Sire.

50 says he only made that decision about Sire after Daphne was labeled a "sex worker" in the Rodney Jones lawsuit against Diddy -- and he claims she retaliated with the open letter to him, posted on IG.

In her post, Daphne scolded 50 for trolling her and Diddy over the lawsuit -- but 50 says the part that really crossed the line for him is when she added, "Let's put the real focus on your true evil actions of raping me and physically abusing me."

According to 50 Cent's suit, her post has more than 22,000 likes, and it's opened him up to a barrage of horrendous online comments. He also says it's severely damaged his business reputation and child custody case.

In the docs, 50 has attached a letter he had his lawyers fire off to Daphne on April 2, demanding a retraction. She not only refused, but 50 claims her lawyers responded by demanding he give Daphne millions of dollars and drop his custody suit in return for her taking down the post.

He calls those demands "clearly extortive."

Now, he's suing Daphne for defamation and seeking damages in excess of $1 million, and a court order to take down her post and refrain from any other defamatory comments.