Surveillance video shows Lane Garrison slapping Ashley Mattingly square in the face after he waited for her outside her apartment -- a much different story from the one Lane told his attorney after his arrest for domestic violence.
TMZ obtained a copy of the building security camera recordings from Saturday ... and the full video (below) shows the "Prison Break" star sitting outside his ex-gf Ashley's Beverly Hills apartment building until she arrives home. When Ashley drives into the building's garage ... Lane is seen stumbling as he runs after her.
Lane and Ashley are then seen walking into the building together, but clearly arguing as they get into an elevator. Ashley seems to be asking Lane to leave ... but when he refuses, she gets off the elevator -- and then things get physical.
Just outside the elevator -- in the apartment lobby -- Lane is seen grabbing Ashley's arm with his left hand ... and then slapping her face with his right. After a few more seconds of struggling ... Lane runs out of the building.
In the final clip ... Lane is seen chucking what we believe is Ashley's mobile phone into the street, and then stumbling up the front stairs and leaving.
Yesterday, Garrison's attorney Harland Braun said Garrison told him he was in the building to dog sit for a friend, and just happened to run into Ashley in the lobby. Braun says Garrison admitted there was an argument, but claimed he did not harm Ashley.
Garrison was arrested for felony domestic violence -- and has yet to post his $50,000 bail. He is also on a parole hold because of his manslaughter conviction.