Diddy is going after a grand jury witness and a lawyer he claims have been spreading lies about him ... accusing them of fabricating stories for media attention.

The disgraced mogul is suing Courtney Burgess -- a man who appeared as a witness for the grand jury -- and Ariel Mitchell, Burgess' lawyer, for spreading what he says are "outlandish claims" and "baseless speculation" to feed the public's desire for "wild lies."

Diddy fired back at some specific claims in the lawsuit as well ... calling out Burgess for alleging he possesses video of the star sexually assaulting celebrities and minors.

The rapper insists no such tapes exist ... and says Burgess and Mitchell are simply telling "blatant falsehoods" or making "recklessly false statements."

Diddy's also firing back at Courtney's claim that he received Kim Porter's memoir directly from her -- saying those closest to Porter like her children and her roommate for more than 20 years have never even heard of him.

As we told you ... Diddy's children previously sent a cease-and-desist letter warning Burgess about releasing the Kim Porter book.

Diddy's suing Burgess, Mitchell and the company behind NewsNation for defamation and is asking for at least $50 million.

We've reached out to Ariel who tells TMZ ... "This is just a pathetic ploy to silence victims and people who stand up for victims. Diddy has time to sue but refuses to accept service for the lawsuits already filed against him. I look forward to countersuing and ensuring the court punishes not only Diddy but also his lawyers who filed this pathetic lawsuit for this frivolous and meritless filing."