They were developed in secret, so the average American did not know there was a program to build them, much less any intent to use them.

The idea of nuclear weapons predated their development, and it was a subject of both science fiction and science journalism up until 1945. There were people who expected their potential. But again, unless they had some reason to believe they were being actively built, they would not have expected them. Their development was extremely fast — the Manhattan Project went from "order to build nuclear weapons" to "actual nuclear weapons used in war" in about 2.5 years, the fastest of any nuclear program ever — and extremely difficult (it required billions of dollars and the labor of hundreds of thousands of people). Unless you had some reason to believe that the US was actually spending the money and resources on that, you would not have anticipated it.

There was no significant opposition from outside the project, because of this state of ignorance. There was some opposition within the project (e.g., the Franck Report, the Szilard Petition), but it was effectively suppressed. The building and use of the atomic bombs was essentially un-democratic in this way; it was managed by a very small group of people, none of whom had been elected other than the President (and, under the early part of the project, the Vice President, in the form of Wallace; Truman was not told about it when he was VP).

I'm currently reading Richard Rhodes' classic The Making of the Atomic Bomb. Although the idea of a large radioactive explosive had been posited in science fiction the general public were not, as you point out, aware of the atomic bomb until it had been used. Details of the atomic tests weren't released until after the bomb had been used against Japan. This story in the New York Times from 06 August was essentially the first the general population heard of the new "cosmic bomb" (as it was briefly called):
http://movies2.nytimes.com/learning/general/onthisday/big/0806.html

Rhodes' book reveals that there was some official opposition to deploying the bomb. Firstly Japan had already been devastated by strategic bombing, with the result that by mid-1945 there were very few cities left to destroy. If there was opposition to the strategic conventional bombing of Japan in the United States and Great Britain I am unaware of it. The original plan was for a strike on Kyoto, but Secretary of War Henry Stimson vetoed that because (a) Koyoto was felt to be of no military significance at all, so there wouldn't even be a tissue of rationalisation for destroying it (b) he liked the place because he had visited there before the war:
https://www.bbc.co.uk/news/world-asia-33755182

Thirdly it was felt that revealing a working bomb to the rest of the world would spark off an arms race, and furthermore than it would put America in the same strategic position as Britain immediately after the development of HMS Dreadnought; the country's overwhelming conventional military superiority would be obsolete, leaving it with a narrow atomic lead.

In the end Stimson it was felt that an arms race was inevitable, and having spend two billion dollars on the bomb, Congress would be unwilling to fund further atomic research unless there were concrete results.