One person was killed Friday in a shooting in the prayer hall of the mosque in the city in northeastern Switzerland, police said.
The suspected killer was arrested at the scene, Saint Gallen cantonal police told AFP, without giving further details about the individual.
Officers were alerted at around 2pm that shots had been fired in the mosque.
At the scene, they detained one man who was in possession of a handgun and found another person lying dead in the prayer hall.
Police are currently trying to confirm the identity of the victim.
Citing members of the Muslim community, local media said the victim was aged 18, and was shot following a dispute between two Albanian families.
The El-Hidaje mosque where the shooting happened is said to be at the heart of the local Albanian community.