MATTHEW FOX DETAINED, ACCUSED OF ASSAULTING WOMAN

Matthew Fox was detained by cops in Cleveland, Ohio Saturday night after allegedly assaulting a woman outside of a bar ... TMZ has learned.

Law enforcement sources at the scene tell us ... the "Lost" star allegedly tried to enter a party bus -- even though he wasn't a guest on the vehicle.

We're told the woman who was driving the bus tried to block Fox from getting on board -- at which point, he allegedly punched her in the breast and the "stomach area."

Fox was handcuffed by an off-duty officer -- and was later detained by police.