Conor McGregor and his entourage just went BESERK at the Barclays Center ... according to witnesses, attacking a bus leaving the arena with several UFC fighters, including Michael Chiesa.
One window was smashed out during the melee -- leaving one fighter, UFC lightweight contender Michael Chiesa, cut up and bloodied.
Fighters were at the arena doing media ahead of UFC 223.
Police sources tell us NYPD got a call for a fight at the Barclays Center involving 25 males -- and we are told a chair was thrown through a bus window.