The group of 50 protesters at the city's Peace Memorial Park, with its leader, Nobuo Takahashi, 72, saying the latest tests were an "unforgivable act" that insults the victims of the Hiroshima and Nagasaki atomic bombings. The bombings, which marked the end of the Second World War, killed 90,000-166,000 in Hiroshima and 60,000-80,000 in Nagasaki, and are the only use of nuclear weapons in war to date.
Mr Takahashi, leader of the Hiroshima Council against Atomic and Hydrogen Bombs, read out a protest letter to President Barack Obama, which said: "We absolutely cannot accept the attitude of sticking tot he possession and development of nuclear weapons, even after (Obama) declared his intention to create a world free of nuclear weapons".
Tomihisa Taue, the mayor of Nagasaki, also sent a letter to Mr Obama denouncing the tests, while Kazumi Matsui, the mayor of Hiroshima expressed his concern over the experiments.
"We call upon the US to withhold from any actions that could generate misunderstandings among those who are sincerely hoping for the elimination of nuclear weapons," Mr Matsui said.
The latest US tests, which took place in November last year and March this year were at the Sandia National Laboratories in New Mexico and used "Z Machine" equipment capable of generating the strongest X-Rays in the world to simulate the fusion that occurs in nuclear reapons. The experiments were used to examine the effectiveness of its nuclear weapons using minute amounts of plutonium.