Every year on Dec 13 an air-raid siren sounds across Nanjing at 10 am.

In the cold, gloomy morning on Thursday, thousands of people lined up in front of the city's Memorial Hall of the Victims of the Nanjing Massacre by the Japanese Invaders to pay tribute to the dead.

On Dec 13, 1937, Japanese troops occupied Nanjing - then the capital of China - and killed more than 300,000 Chinese civilians and disarmed soldiers over about six weeks.

On the 75th anniversary of the Nanjing Massacre on Thursday, Chinese and Japanese mourners gather in the Memorial Hall of the Victims of the Nanjing Massacre by the Japanese Invaders in Nanjing, Jiangsu province, to pay tribute to those who died in the atrocity. [Photo by Li Ke / for China Daily]

To mark the 75th anniversary of the massacre, a series of memorial activities was held in Nanjing, including a candlelight vigil and gatherings in places where group slaughters occurred.

On Thursday morning, nearly 200 monks from China and Japan prayed for world peace. About 9,000 people, including some from the Republic of Korea, India and the United States, then attended a ceremony at the memorial hall.

At the ceremony, wreaths were presented, a moment of silence was observed and every person bowed three times to the victims. Chinese and Japanese choirs then sang three songs about peace.

Yang Weize, Nanjing's Party chief, said at the ceremony that thinking about history makes people value peace more.

"The massacre was one of the darkest periods in the history of modern civilization. It caused unforgettable pain to every Chinese person, and it should not be forgotten by any nation in the world, including the Japanese," Yang said.

A survivor of the Nanjing Massacre, Gu Xiulan, traveled for more than an hour to attend the ceremony at the memorial hall. Her son and daughter accompanied the 88-year-old, who is in a wheelchair.

Gu's son, Long Jinyuan, said that his mother has cried a lot in recent years when she remembers her deceased relatives.

"We know what kind of pain the war can bring," said Long. "As a family member of one of the victims, I hope that Japan will learn from other countries that correctly acknowledged their war crimes."