On Saturday night, tens of thousands of mostly young people flocked to Itaewon in Seoul to celebrate Halloween.
Images of Itaewon subway station before the tragedy show large crowds gathering in the area, known for its nightlife, bars and restaurants.
The lively district is a warren of steeply sloping, twisted alleyways on either side of the main road.
An estimated 100,000 people packed the streets for the first virtually unrestricted Halloween festivities in three years. Witnesses said police had trouble controlling the crowds.
Just after 10pm, chaos erupted. Witnesses reported seeing crowds surging in different directions and people losing their footing on the slope, causing a domino effect.
A narrow alley near the Hamilton Hotel become flooded with people. Korean news agency Yonhap said the back alley is a downhill, 40-metre path, that links a busy restaurant district with a main street. About six adults can barely pass through the alley at the same time, according to Yonhap.
People fell and knocked others down, piling up on one another, unable to move.
The first calls to emergency services were made around 10.15pm. Fire authorities initially received dozens of reports from people in the area with breathing difficulties, Yonhap said.
Social media footage showed some people trying to scale the sides of the buildings to escape the increasing pressure, as others shouted, cried or cursed.
Witnesses said the streets were so densely clogged with people and slow-moving vehicles that it was practically impossible for emergency workers and ambulances to swiftly reach the alley near Hamilton Hotel.
Witnesses, including local shopkeepers, said the number of people attending festivities in Itaewon appeared far higher than in previous years.
The country has started a week of national mourning. On Monday, people wept in grief, prayed and placed flowers at a huge official altar set up in central Seoul for victims – mostly young women – of the tragedy.
White chrysanthemums, drinks and candles were left at a small makeshift altar off an exit of the Itaewon subway station.
Reuters, Associated Press and Agence France-Presse contributed to this report.