Police have charged a 31-year-old Red Hill man following an incident at a bar in South Brisbane last night.

It will be alleged that at around 9pm the man has approached a 19-year-old man in a bar on Montague Road and has taken his mobile phone and pushed him.

It is then alleged the 19-year-old man attempted to stop the man but the man has grabbed him and forced him to the ground before leaving on foot.

The 19-year-old has then later seen the man in the area and approached him but was then threatened by the man with a knife.

Police have observed the man a short time later and have arrested him.

The 31-year-old man was charged with one count each of armed robbery and going armed so as to cause fear. He is due to appear in the Brisbane Magistrates Court tomorrow.