From casual mentions in memoirs and in Orwell, I know that there were left-wing publications, meetings and protests against Hitler and concentration camps outside of Germany, but I'm not having a lot of luck searching for more information about this. Do you have any recommendations or search terms?
Orwell mentions a book or pamphlet (but doesn't give a title) published in the UK about the early anti-Jewish campaigns/stripping people of rights/etc. I expect that a lot of this stuff was relatively ephemeral, just judging from how ephemeral a lot of US activist materials that I literally remember turn out to have been.
A lot of times, you feel like it just isn't doing any good to protest or volunteer against detention camps or for immigrant rights - the forces aligned against us are too powerful. And similarly, it's obvious that whatever activists did to try to draw attention to Naziism didn't do much good. But I still think it's worthwhile to leave a trace in history so that future people will know that not everyone was just cool with detention camps and genocide.