Startling video footage shows the moment a sheriff's deputy boards a bus and punches a woman across the face.
With the woman struggling to escape as he pins her arms to her side, the cop pulls back an arm and smacks her so hard she loses her balance.
The video was captured on a mobile phone by a bus passenger in Los Angeles, California who described the woman as having 'special needs'.
Mr Green said the officer then demanded he hand over the mobile phone and threatened him with arrest - but he refused.
'I think they would've tried to cover it up,' he told KNBC.