China marked 70 years since Japan's Nanjing massacre on Thursday, invoking memories of the atrocity to remind Tokyo that the wartime past remains a bitter backdrop to an improving relationship. Sirens wailed, calling citizens to silence, a bell tolled, and tens of thousands of people, including frail survivors, gathered for the reopening of a newly expanded massacre memorial in the former national capital in eastern China. The six-week wave of killing by Japanese soldiers after Nanjing fell was among the bloodiest episodes of Japan's invasion of China. Official Chinese accounts say 300,000 were killed. For China, how Japan remembers the "Rape of Nanking" -- as the city was then called in English -- has become a test of how contrite its neighbor is about its brutal occupation of much of the country from the 1930s up to 1945. Aged survivors came out to remind the world of the event. Chen Fubao, 75, clutched a black-and-white photo of his father, who was killed in the slaughter. "We hope that the Japanese government, especially those in the nationalist factions, will admit the truth in history and learn from the Germans," he told Reuters. "They should not cover up their crimes any more." Beijing and Tokyo have been moving in recent months to ease long-running tensions over history, territory and energy and commemorative propaganda has avoided harsh words about Japan's current leaders.