Ryan Garcia Disses Errol Spence Jr. Using Kendrick Lamar's 'Not Like Us' Beat

Ryan Garcia launched an all-out social media attack on Errol Spence Jr. on Wednesday ... challenging the fellow boxing star to a fight -- and doing his best Kendrick Lamar impression in the process.

The feud sparked after one of Spence's fans said they'd like to see him take on King Ry as his next opponent ... to which Garcia unloaded a number of fiery responses, including a post claiming he already told his camp to make it happen.

Garcia even threw out a date and a place ... saying he'll be ready to throw hands on August 10 at AT&T Stadium in Texas. He also seemed to be down with Spence's pitch to fight between 154 and 160 pounds (super welterweight or middleweight) ... despite that being at least 14 pounds more than what he was supposed to come in at ahead of the Devin Haney bout.

The 25-year-old pugilist then hopped on Instagram Live to call out Spence with a freestyle rap ... choosing K.Dot's "Not Like Us" as his instrumental of choice.

Of course, the beat has gone super viral amid Lamar's intense beef with Drake ... but if you were hoping for some high-quality bars with Garcia's rendition -- well, you'll be disappointed.

Garcia dropped some scathing allegations in his brief cover ... claiming Spence is broke and his girl has been hitting him up in the DMs.

As for Spence, he called Garcia "coward as f***" for his performance against Gervonta "Tank" Davis last year ... saying he'd be nothing but a tune-up fight.