Police have arrested a man after they say he sexually assaulted his 4-year-old niece on a bus in Queens. 

Authorities say 30-year-old Jesus Rodriguez Rojas is charged with sex abuse and acting in a manner injurious to a child. 

Investigators say Rojas was riding the Q53 bus with the girl when he inappropriately touched her. 

Sources say witnesses saw Rojas touching her and started yelling at him. 

The child was taken to the hospital for observation.