At least another 82 were injured in the incident in the Itaewon nightlife area which was holding its first unmasked Halloween celebrations since Covid.
Reports describe a desperate scene of people caught up in the crush piling on top of each other.
Most of the dead were teenagers or in their 20s. Nineteen were foreigners.
The cause of the disaster is still being established.
After holding an emergency meeting, South Korea’s President, Yoon Suk-yeol, ordered a task force to be set up to help treat the injured. He also launched an investigation into the cause of the crush.
An estimated 100,000 people came to celebrate there on Saturday to mark the first Halloween since the start of the pandemic where gathering sizes were not limited and people did not need to wear masks outside.
The crush appears to have started in a narrow, sloped alleyway that was overcrowded with people.
Pictures and videos on social media show the alley was so densely packed, people were unable to move. One video shows people struggling to breathe. In another, emergency responders try to pull out people from what appears to be a pile of bodies. Cries of distress can be heard.
Bodies were lined up along the street covered in blue blankets. Others were carried, lifeless, into ambulances. Members of the public desperately administered CPR to those who were lying unconscious, along with hundreds of emergency workers who were sent in from around the country to help.
Relatives and friends of missing people have been turning up at the scene this morning looking for clues to reveal whether their loved ones were there.
But the bodies have been moved from the street into a gymnasium, for family members to come and identify them. With so many victims, this is expected to take time and is the authorities’ immediate priority.
Jeon Ga-eul, 30, was having a drink at a bar at the moment the crush began.
“My friend said: something terrible is happening outside,” she told AFP news agency. “I said: what are you talking about? And then I went outside to see and there were people doing CPR in the street.”
A doctor who administered first aid at the scene said that when he had first started performing CPR there were two victims but “the number exploded soon after, outnumbering the first responders”.
Social media messages posted earlier in the evening showed some people remarking that the Itaewon area was so crowded that it felt unsafe.
One witness, Park Jung-Hoon, told Reuters news agency that big crowds were normal for Christmas and fireworks celebration but “this was several ten-folds bigger than any of that”.
A local journalist said that an emergency broadcast had been sent to every mobile phone in the Yongsan District urging citizens to return home as soon as possible due to “an emergency accident near Hamilton Hotel in Itaewon”.
The attention will likely shift to the safety standards and crowd control measures in place at these occasions. President Yoon has already called for a review of the safety of festivity sites.
Several world leaders have expressed their condolences. British Prime Minister Rishi Sunak and French President Emmanuel Macron said their thoughts were with the people of South Korea.
The EU’s chief diplomat, Josep Borrell, said he was deeply saddened and the US national security adviser, Jake Sullivan, pledged American support.