On assuming power in 1933 the first people the Nazis targeted for arrest and imprisonment were political opponents – primarily communists, trade unionists and social democrats.
Dachau, the first Nazi concentration camp, was built in March 1933 to imprison political opponents. The Communist Party (KPD) was banned in March 1933, trade unions were disbanded in May and the Social Democrats (SPD) abolished in June. Leaders of these parties and unions were arrested or fled into exile.
By the end of 1933, there were almost 27,000 people imprisoned in concentration camps, and the majority of these were political prisoners.
For more information, read I am healthy and I feel fine, the collected letters of Marian Serejski who was held as a political prisoner in Auschwitz.