Darwin was already known in scientific communities for his Voyage of the Beagle, and had already been admitted as a Fellow of the Royal Society. He also had many serious friends whose vouching for him mattered (e.g. Lyell and Hooker). So all of that helped a lot in making sure his book was taken seriously.

The idea of evolution was not new; aside from Lamarck, within Britain it had already been popularized heavily by the anonymously published Vestiges of the Natural History of Creation, which for many years after Origin of Species outpaced the latter in terms of circulation. Vestiges was more loosely written and poorly sourced than Darwin, and not regarded as a "serious" book by "serious" people, but was a widely-known referent (including to Darwin himself). Darwin's book was initially seen as "Vestiges but serious," you could say. This was not quite right (there are significant differences in every aspect of the two works), but the perception helped Darwin in the public eye, initially.

Most frustrating to Darwin is that the specific novelty of his theory, namely that evolution was caused by natural selection, was largely originally overlooked; he was read as giving a far more serious treatment of evolution as a concept than the previous writers had been. It took some time and some discernment for people to make the important distinctions.

Darwin's work circulated popularly not just by word of mouth and reputation, but through an extremely popular book club (Mudie's Circulating Library), which was sort of like a 19th-century Netflix (the non-streaming, original Netflix). This got him a much broader audience than he would have already had. His previous fame for The Voyage of the Beagle probably helped with this — it was a fairly popular read (unlike, say, his later work on barnacle reproductive organs).

Reactions were varied depending on what one means by "people." Many scientific peers dismissed it as Lamarck/Vestiges tarted up. Some people rejected it on religious grounds as they would all evolutionary theories. Marx remarked that it was obviously just capitalism dressed up in biological language. Some of course embraced it. And so on. It was contentious, to be sure, and an opportunity for people to have arguments over things they were already somewhat inclined to argue over.

Anyway — this is just an overview of the context. It's an immense topic. Janet Browne's Darwin: The Power of Place is a favorite biography of mine because it covers this period very well as well as the context and reception. The Vestiges vs. Origin graph, and much else on Vestiges, is from Jim Secord, Victorian Sensation.