Manny Pacquiao -- Legendary Trainer Freddie Roach KICKED IN THE CHEST ... In Pre-fight Fight

Freddie Roach -- perhaps the most famous trainer in boxing history -- took a boot to the chest this morning during an altercation at a boxing gym in Macau ... and it was all caught on video.

Roach had just arrived to the gym to begin a training session with his fighter Manny Pacquiao -- when he got into a verbal spat with the people training Brandon Rios ... the guy Manny is set to fight this weekend.

Roach wanted the Rios camp out of the gym so Manny could begin training -- and the Rios camp didn't want to budge (Manny was not at the gym during the incident).

You can hear Freddie call one of Brandon's trainers a "piece of s**t" -- and trainer Alex Ariza responds by planting his foot in Roach's chest.

FYI -- Roach suffers from Parkinson's Disease.

Security scrambled to get between the two ... as the expletives continued to fly. At one point, Ariza mocks Roach's disease and calls him a "f**king joke." He also hurled a gay slur.

During the melee, Roach spewed a Mexican slur aimed at one of the Rios' trainers.

Roach told security he wanted Ariza arrested -- but it's unclear if that ever happened.

What's more dramatic ... Ariza used to work for Freddie -- and it's obvious there's still bad blood from when he left Roach's camp.