One of the prominent critics of the bombings was Albert Einstein. Leo Szilard, a scientist who played a major role in the development of the atomic bomb, argued "If the Germans had dropped atomic bombs on cities instead of us, we would have defined the dropping of atomic bombs on cities as a war crime, and we would have sentenced the Germans who were guilty of this crime to death at Nuremberg and hanged them."
Their use has been called barbaric as several hundreds of thousands of civilians were killed, and the choice of areas heavily populated by civilians. In the days just before their use, many scientists (including Edward Teller) argued that the destructive power of the bomb could have been demonstrated without the taking of lives.
It has been argued that the use of atomic weapons against civilian populations on a large scale is a crime against humanity and a war crime. The use of poisonous weapons (due to the effects of the radiation) were defined as war crimes by international law of the time. Some have argued that Americans should have done more research into the effects of the bomb, including radiation sickness and the terrible burns that followed the explosion.

The information I have posted comes from Wikipedia sources