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A chill night out turned into chaos for James McAvoy , who was randomly sucker-punched in the back of the head by a stranger at a Toronto bar!

Sources with direct knowledge tell us the actor -- in town for the premiere of his directorial debut California Schemin’ at TIFF -- got the shock of his life Monday at Charlotte’s Room... when a wasted guy being tossed out clocked him from behind. Gallery Launch Gallery Getty

We’re told McAvoy shot up and was like WTF?! ... but he walked away totally fine. No injuries, no medics... and no arrests, since cops were never called.

It’s unclear if the drunk even knew he’d just swung on James Mcvoy, the actor -- but McAvoy kept it cool, barely engaging after the hit and deescalating the whole thing.

Sources add James stuck around aft

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