EXCLUSIVE

Frank Dillane -- star of "Fear the Walking Dead" and a 'Harry Potter' flick -- allegedly got physical with a CBS security guard, and got hauled off the backlot by cops ... TMZ has learned.

Our law enforcement sources say Dillane, who plays Nick Clark on 'Dead,' tried to breeze through a security checkpoint on Sunday night. Guards tried to stop him, especially because 'Dead' doesn't even shoot on the lot. At least one guard described his behavior as "erratic."

We're told Dillane didn't dig all the attention, things got heated and he ended up striking one of the guards. That was the last straw for the security force, which detained the actor with a citizen's arrest.