Before those concentration were built (and even afterwards) 'non-conformists' were taken into "Schutzhaft" (
protective custody) in normal prisons or specially altered buildings.
This could happen "legally" due the "Reichstagsbrandverordnung" from Fenruary 28, 1933 [a special by-law after made after the burning of the Reichstag) in conjunction with the "Notstandsartikle" (state of emergency constitutional article) 48, 2 of the Weimar constitution.
Such places looked like this one
Those Communists and Social-Democrats were later tranferred to the concentration camps.
The leading journalist of a smaller newspaper (a Communist or at least very left Social-Democrat) was imprisoned there and later "shot on the run" on the transport to a KZ: there are letters published, where he said, hew wanted to go into the KZ to report from there.
He was Jewish as well, but in those days that wasn't a reason for getting imprisoned (but not long afterwards).