SEAN YOUNG Arrested for Oscar Party Fight Allegedly Slapped Guard

6:30 AM PST -- Now we know what allegedly happened.  We're told Sean was standing near the entrance of the Governors Ball, taking pictures with friends when security guards realized she didn't have a ticket so they asked her to leave.  We're told Sean left but came back shortly thereafter, and when security asked her to leave again, a guard may have placed his hand on her arm when Sean allegedly slapped him.  She was then placed under citizen's arrest for battery.

6:10 AM PST --  We just got video of Sean leaving the LAPD Hollywood substation, where she blamed her arrest on the Academy's lawyer.

1:45 AM PST --   We just spotted Sean Young leaving the police station. She claims she was waiting with friends to enter the Governors Ball when an Oscar official suggested security make a citizen's arrest. She did not say why the official made that suggestion ... or mention any physical encounter with the security guard. 

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And the handcuffs go to ... Sean Young -- who was busted by police at the officialOscar after party for allegedly getting physical with a security guard ... TMZ has learned.
Sources tell us ... Young was attending the Governors Ball -- the annual schmancy Oscar party thrown by the Academy itself -- when she allegedly got into a dispute with a security guard and made contact.

We're told the "Ace Ventura: Pet Detective" star was placed under citizen's arrest by security -- until LAPD arrived to take her into custody. Young was booked for misdemeanor battery.

In the hours before her arrest, Young was taking pictures with some of the biggest stars in the world ... including Angelina Jolie, Brad Pitt and Sandra Bullock.