This is the first time China's held a national memorial day to mark the Nanijng Massacre of 1937.
Beijing says Japanese troops killed 300, 000 in what was its capital at the time.
A post-war Allied tribunal put the number of dead at just under half that.
Some in Japan deny the bloodshed ever happened at all.
Ties between China and Japan have deteriorated over the past year after Japan's prime minister visited a shrine honouring war criminals among Japan's war dead.
A spat over islets in the East China Sea has added to the problems.
Speaking at Saturday's memorial in Nanjing, China's President Xi Jinping said history shouldn't be forgotten but nor should the future.
(SOUNDBITE)(Mandarin) CHINESE PRESIDENT XI JINPING SAYING:
"Forgetting history is a betrayal and denying a crime is to repeat a crime," HE SAID. "We should not hate the people just because a small minority of militarists set off an invasion and war. The crime of war is on the
