Some 81 members of the church were gathered in their Kingdom Hall on Dunlopstraße in the city's Sennestadt district when a congregation member noticed the masked elderly man enter the vestibule waving the machine gun around 8:45 pm on Thursday.
The witness quietly informed the congregation and directed them to escape the building via emergency exit, a police statement said.
Meanwhile the man entered and left the hall before attempting to return to his vehicle, an Opel Astra parked some 50 metres away, waving his gun along the way.
But before he could leave the scene, two of the congregation members overcame the man from behind. Police arrived shortly thereafter to arrest him.
Police in the state of North Rhine-Westphalia are investigating the man's motives, the statement said.