Hey Tatjana, I didn't have much luck searching for vegan organizations in Bremen, but found this website that might be a good start. http://www.vegan.de/

Of course, I only searched in english, so try searching in german. I bet you'll find more than one organization where you can work. If not, try doing some solo activism. Here's an old thread where we shared some ideas on this:

Vegan Education - Let's Get Active

Cheers!

Many thanks for the answer. I am on my own path to become from vegetarian into a vegan, but have some further questions.
dolphin slaughter -> I am never going to visit Japan even when having some German friends living there. Leather industry -> dumped my leather coats into the dustbin and had a long long search until I found a jacket NOT made in China (hell no, I am not going to support this country when I can help it). Pets -> adopted some from perreras (killing stations) in Spain and am now the companion of seven cats (all neutered or on their way to become neutered because still too young). My question is, where on Earth did this happen with the dog being thrown into the trashcompactor? Turkey, Irak, Iran? I am going to give that blasted part of earth living hell wherever I can give it.