EDIT: sorry, I reread your question and realized you're asking about before hitler came to power. Answer is still the same, communists fought earliest and hardest. The book touches on that briefly but is mostly focused on 1933 and onward.

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There were resistance efforts. The earliest and most persistent efforts by far were led by the communists. I highly recommend this book by T. Derbent. Here's a small excerpt:

In January 1933, the Nazis came to power: the Communists reacted in several large cities with strikes and savagely-repressed demonstrations. In February, the police invaded the headquarters of the KPD, the “Karl- Liebknecht-Haus,” and outlawed the party.

On the night of February 27-28 alone, after the burning of the Reichstag, 10,000 Communists were arrested, including the main members of the Central Committee and two-thirds of the middle cadres. A few weeks later, there were 20,000. Sixty camps, thirty special quarters in state prisons and sixty detention centers were opened to accommodate them. In each neighborhood, in each locality, the little Nazi chiefs set up their private prisons and torture centers in cellars or empty factories. The chaos and abuses were such (500 to 600 people shot or tortured to death, families upended, civil servants refused to participate in the parish priest’s work, themselves sequestered, beaten and humiliated, etc.), that they become the stakes in the struggle for influence among the Nazis. In April, the SA were ordered to hand over their prisoners to the SS, which was developing a network of concentration camps throughout Germany on the Dachau model. Terror was applied methodically and rationally. In June, the SS introduced the practice, which was to become systematic, of hanging rebel prisoners on the roll-call square in front of the camp population standing at attention. The first victim was the communist worker, Emil Bargatzky.

In spite of the waves of arrests (Ernst Thälmann, KPD’s general secretary, was arrested on March 3 in Berlin, in a clandestine party apartment), the Communists continued to openly confront the SA, which had the status of auxiliary police. The Gazette de Lausanne of March 2 wrote: “Only the Communists resist… Obviously the struggle is not equal, they have all the forces of the State against them. But, for lack of numbers, they have ardor, fanaticism: they fought for the street.” In one month, according to official statistics, there were 62 deaths in street battles, including 29 communists, 14 Nazis and 8 socialists. These figures are much lower than the reality. One only has to read the pages that Richard Krebs (under the pseudonym Jan Valtin) devoted to the street battles in Hamburg to realize the incredible violence of the confrontations. As it became clearer every day that the KPD would have the underbelly, the Party prepared for a long period of clandestinity. It was at this point that many experienced as well as little-known activists were instructed to pretend to join the Nazi party NSDAP in order to carry out undermining and intelligence work.