Police have charged a 12-year-old boy with robbery while armed, allegedly committed in St Marys on Sunday.
Police said a boy and another, older male, chased a 15-year-old boy from a St Marys youth centre about 4pm.
They alleged he was robbed of his mobile phone, after being tackled to the ground, punched and threatened with a knife by the older male.
St Marys police started investigating and later arrested a 12-year-old boy in Mount Druitt.
He was charged with robbery while armed and appears in a children’s court on September 1.
Police are appealing for help to locate the other male, described as about 16-years-old and of African appearance.
Anyone with any information is urged to contact St Marys detectives on 9677 7501 or Crime Stoppers on 1800 333 000.