Sirens wailed in China's Nanjing city, 75 years after Japanese troops embarked on mass killing and rape, as a modern-day territorial row between the two saw Tokyo scrambling fighter jets.
The two countries - the world's second and third-largest economies - have extensive trade and business links, but the weight of Japan's wartime atrocities still bears heavily on their relationship.
Nearly 10,000 people sang the Chinese national anthem on Thursday at a commemoration at the Nanjing Massacre Museum, as soldiers in dress uniforms carried memorial wreaths across a stage and officials urged remembrance of the past.
Beforehand an elderly woman cried as she placed flowers by the names of family members listed among the victims on a grey stone wall, and a group of Chinese and Japanese Buddhist monks chanted sutras to pray for world peace.
"We are here to recall history, grieve for compatriots who suffered and died, and educate the people ... about the lessons of history," said Nanjing Communist Party chief Yang Weize, the only government official who spoke.
China says 300,000 civilians and soldiers died in a spree of killing, rape and destruction in the six weeks after the Japanese military entered its then capital on December 13, 1937.
Some foreign academics put the number of deaths lower, including China historian Jonathan Spence who estimates that 42,000 soldiers and citizens were killed and 20,000 women raped, many of whom later died.
On its website, the Japanese foreign ministry concedes only that "the killing of a large number of non combatants ... occurred" and says that "it is difficult to determine which the correct number is".
Some ultra-conservative Japanese politicians dispute that atrocities ever took place in Nanjing.
Fewer than 200 survivors remain, according to Chinese estimates. One of them, Li Zhong, 87, said he can never forgive, recalling how people had to restrain a man who grabbed a knife to kill Japanese soldiers after his wife was raped.
"There are fewer and fewer of us survivors every year," he said. "We must never forget history."
Kai Satoru, the son of a Japanese soldier who served in China, was among the hand-picked audience, which included Chinese students, soldiers and government officials, as well as Japanese NGO representatives.
"I am here to admit the crimes. They (Japanese soldiers) competed to kill people," he said.
Two Chinese men assaulted a Japanese journalist reporting on the ceremony by kicking him once and attempting to choke him, before several uniformed police intervened to stop them.
The journalist, Shanghai bureau chief for Kyodo News Tomoji Tatsumi, told AFP he was not seriously injured.
The Japanese government has reported at least seven assaults of its nationals in China since a political row began in September over disputed islands in the South China Sea.
The 75th anniversary has taken on added meaning given the poor state of bilateral ties.
Japan on Thursday scrambled F-15 fighters after a Chinese state-owned plane entered airspace over islands claimed by both countries.
Chinese government ships have moved in and out of waters around the islands - known as the Senkaku Islands in Japan and the Diaoyu Islands by China - for more than two months.
But it was the first time that Chinese planes have ever intruded into Japanese airspace, according to the defence ministry in Tokyo, while China defended its right to overfly what it says is its own territory.