T.I. -- Shoving Match at Vegas Pool Party ... Hours Before Mayweather Brawl [VIDEO]

T.I.'s temper was already revved up just hours before he got into a violent brawl with Floyd Mayweather on the Vegas Strip -- when security at a crowded pool party DENIED his ass ... and it's all caught on video.

Witnesses tell TMZ T.I. had shown up to Liquid -- the pool party at Aria where Nicki Minaj was performing last weekend -- but he tried to side step security with his posse.

Security wasn't having it ... insisting T.I. had to be searched just like the normies entering the party.

The real problem -- we're told the door staff didn't recognize Tip ... and once they did realize it was him they let him right into the event.

But the standoff was intense -- Vegas PD even got involved -- and after watching the video, you'll understand why T.I. had a short fuse when he faced off against Floyd later that night. Pretty nuts.