EXCLUSIVE

Paris 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.

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*cking fight you."

-- "I am going to f*cking kill you."

-- "I will f*cking rip through you."

-- "I will f*cking own anyone on this flight; they are f*cking peasants."

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.

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.

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."

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