I know exactly when I read this story, in the spring of 1968, when I was in Germany to improve my german. Of course it could have been written any time before. I actually read it in german. But I was not really so fluent in german, so I deliberately chose to read this rather short book with the rationale that, being a translation from english, it would have shorter sentences. And it worked, I managed to read it to the end.
I'm almost sure there was only this story in the book, but it was a short one, not a novel, just a novella and not a very long one at that. In hindsight, I would say it was for "Young Adult", but of course I was a young adult then.
The beginning was perhaps inspired by Asimov's "Pebble In the Sky". There was an incident in a nuclear power plant, and a whole community (not a big city, but a town, not too small) was sent into the future. But the Earth they reach is empty. I think the power plant came with them, because I think they still have electric power. That's how they realise that noone else is there because they don't receive any broadcast from anywhere. They use their own radio station to contact other towns or cities but get no immediate answer. But after a few days one spaceship, or maybe more, land near them. Their calls for help have been heard. Out of the spaceship came all kind of species, many alien ones, but also humans. The humans are dominant but the others are not slaves or anything, just like "junior partners". At first the people from the past are very afraid of the strange-looking aliens, but soon find them more friendly than the humans of the future.
Another common point with "Pebble In the Sky" is that these humans from space are not very warm for the Earthlings from the past. The reader at some point finds out that in the past, the humans that left Earth to become "Spacers" despised the ones who stayed on Earth. There was some bitter happenings. And mankind died on Earth. Even though these people from the past were before space travel and thus ancestors to all humans, the spacer humans from the future despise them.
Sure, they came to help, not to kill, but it appears that they want to keep them from modern technology, just help them to barely survive on the empty planet. It is the other species from space who join with the humans from the past. I don't remember any fight or bloodshed. It is just a matter of groups of influence, the spacer humans from the future finding everyone else against them and having to give in and give equal status to all.
Well, this is if I remember the end correctly. The elements which are really remember is the aliens and the humans form the past joining forces against the humans from space. I don't remember violence, but maybe I am wrong.