Tsuboi was on his way to engineering school in 1945 when the first nuclear bomb attack was launched by the United States, turning the bustling metropolis into an inferno.

"I suffered burns all over my body," he told AFP in 2016. "Naked, I tried to run away for about three hours on August 6 but finally could no longer walk."

Then aged 20, he picked up a small rock and wrote on the ground "Tsuboi dies here", before losing consciousness and waking up several weeks later.

He later developed cancer and other diseases but became a prominent advocate for atomic bomb survivors and a lifelong campaigner for a nuclear-free world.

"I can tolerate hardships for the sake of human happiness. I may die tomorrow but I'm optimistic. I will never give up. We want zero nuclear weapons," he said.

Tsuboi was among a handful of Hiroshima survivors who met then US president Obama when he visited the city in 2016.

He smiled broadly as he shook Obama's hand, with the two men conversing for upwards of a minute. "I was able to convey my thoughts," a satisfied Tsuboi said afterwards.

Tsuboi "passed away on Saturday due to anaemia", an official from Nihon Hidankyo -- a group that represents survivors of the atomic bombings of Hiroshima and Nagasaki, of which Tsuboi was a key leader -- told AFP.

There are 127,755 survivors of both attacks still alive and their average age is 84, according to the health ministry.

Around 140,000 people died in the bombing of Hiroshima, a toll that includes those who survived the explosion but died soon after from radiation exposure.

Three days later the US dropped a plutonium bomb on the port city of Nagasaki, killing about 74,000 people and leading to the end of World War II.