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(TMZ) - Hilton's younger brother Conrad Hilton was just arrested in L.A. for allegedly going insane on an international flight back in July ... and getting violent with flight attendants ...  TMZ has learned.

TMZ reports, according to officials, multiple witnesses on the plane say the 20-year-old went berserk on a flight from London to L.A. ... and began screaming and threatening flight attendants.  

According to witnesses, Conrad was heard yelling the following things during his meltdown: 

-- "If you wanna square up to me bro, then bring it and I will f***ing fight you."
-- "I am going to f***ing kill you."
-- "I will f***ing rip through you."
-- "I will f***ing own anyone on this flight; they are f***ing peasants'."

According to TMZ, witnesses say ... at one point during the flight, Conrad was ferociously punching the bulkhead of the plane ... 10 centimeters from a flight attendant's face. 

Witnesses also say they could smell weed seeping out of the bathroom ... and believe Hilton was the culprit.  

TMZ reports Conrad was allegedly so out of control, children on the plane were crying.  

At one point, Conrad fell asleep ... and the captain authorized crew members to restrain him to his seat for the descent. Conrad was ultimately handcuffed to his seat, according to TMZ.

Witnesses told officials Conrad grabbed a flight attendant's shirt and said, "I could get you all fired in 5 minutes. I know your boss! My father will pay this out. He has done it before. Dad paid $300k last time."

According to documents obtained by TMZ, Hilton admitted to taking a sleeping pill before the flight and then bragged that he "buried" the flight attendants. 

"I told all of them I could get all of their jobs taken away in less than 30 seconds."  

TMZ spoke with Hilton's attorney Robert Shapiro -- yes, that Robert Shapiro -- who tells TMZ Conrad took a sleeping pill before the flight and he wasn't himself.

Shapiro says there are numerous news reports and cases of people experiencing adverse effects, including aggressive outbursts ... and he's suggesting that's what happened here. 

Photo: Splash News