There were no specific warnings that Hiroshima, Nagasaki, or Kokura (the primary target for the second bombing mission) would be bombed. Hiroshima and Nagasaki have been asserted to be on lists of cities that had gotten generic "we are bombing cities" leaflets, but frankly I've never seen evidence of that. They certainly got no specific warnings about the atomic bombings.

But there is one interesting thing here that is not well-known. Citizens in Hiroshima had, in fact, worried that they might be getting bombed soon (not atomic bombed, of course, but bombed). Why? Because they had a military base there and they had been conspicuously not bombed while the rest of Japan was being turned to ashes. So they figured something was coming their way.

From the Preliminary Report of Findings on Atomic Bomb Investigating Groups at Hiroshima and Nagasaki, late November 1945:

Just previous to the bombing of Hiroshima plans were being made for the evacuation of unnecessary persons. The day of the bombing 40,000 extra people were brought into the center of the town for instructions on these evacuation plans.

So that intuition didn't help them very much. If they had been given warning, maybe they'd have gotten out of there earlier. As it was...

Nagasaki had been conventionally bombed several times already during the war, as recently as August 1st, 1945. Many of its "non-essential" people had already been evacuated to the countryside. This is one of the reasons that the death toll was fewer than at Hiroshima (there were geographical reasons as well). There were still plenty of civilians, though, especially in the area where the bomb went off (which was mostly civilian).

Here is an earlier answer that might be helpful. The short answer is "no-one" and that daminteresting.com is a terrible source.