12:18 PM PT -- TMZ Sports now has video showing Conor picking up a dolly and smashing the side window of the bus.
We spoke with Dana White ... who tells us Michael Chiesa was "cut to pieces" in the attack and is currently hospitalized for his injuries. It's unclear if he will fight as planned at UFC 223.
White says Rose Namajunas was also on the bus and very shaken up after the incident -- telling us she almost broke her hand.
We asked Dana if he will fire Conor as a result of the incident and he told us, "This is bigger than us firing him."
An object was thrown through one of the bus windows -- and the damage left one fighter, UFC lightweight contender Michael Chiesa, cut up and bloodied.
Unclear who was responsible for shattering the side window but it's clear people on the bus suspect it's Conor.
A member of Conor's entourage then grabbed a chair and threw it at the front windshield of the bus. Conor grabbed a chair and tried to attack as well, but was restrained.
Fighters were at the arena doing media ahead of UFC 223 ... and McGregor was videotaped by several witnesses running around the arena before the incident.
Police sources tell us NYPD got a call for a fight at Barclays involving 25 males ... and EMS was called to the scene to treat injuries.
Unclear what may have triggered the incident, but Khabib did get into a heated confrontation with Conor's good friend and training partner, Artem Lobov, Tuesday at the UFC fighters' hotel in New York.
Story developing ...