Conor McGregor Goes '12 Hard Rounds' with Boxing Champ, Leaves Bruises

Conor McGregor put in some violent work Tuesday night -- going "12 hard rounds" with boxing champ Paulie Malignaggi ... and left some marks in the process.

The two got in the ring in Vegas for a full speed workout session to prep for the August 26 superfight with Floyd Mayweather -- and the former WBA welterweight champ tells TMZ Sports, "We both gave each other really good work."

Malignaggi says McGregor been making steady improvements in the ring since the last time they fought 12 days before ... and says he's landing real shots. You can tell by Paulie's face.

As for Conor's power, Paulie says it's time for a reality check.

"Conor doesn't have bad power. It's not 'Oh my god' power ... but it is respectable power."

"Every time I say this people get offended. It's like they want you to lie. It's like they want you to say he has this incredible power like I've never felt anything like it before."

"It is power to where if it hits you, it's gonna get your attention and it may even put you down if you're wearing those smaller fight gloves."