Bells tolled and thousands bowed their heads in prayer in Hiroshima on Thursday at ceremonies marking the 70th anniversary of the world's first atomic bombing.

At 8:15 am, the exact time the bomb exploded on Aug 6, 1945, the crowd stood for a moment of silence in the heavy summer heat while cicadas shrilled, the Peace Bell rang and hundreds of doves were released into the sky.

Prime Minister Shinzo Abe, who in a speech at the ceremony called for abolishing nuclear weapons, repeated his view that legislation moving Japan away from its pacifist Constitution was essential to ensure the nation's safety.