Prime Minister Han Duck-soo has vowed a "thorough investigation" into the cause of the tragedy
Funerals for some of the 154 victims have begun taking place
The victims - mostly in their 20s and more women than men - had thronged a popular nightlife district for Halloween
It was the first outdoor no-mask Halloween event with no limit on numbers since the pandemic
The crush began in a narrow alley, but officials don't yet know the cause or the circumstances that led to it
The foreign ministry says 26 foreign nationals are among the dead, Reuters reports
Video filmed at the scene in Itaewon shows people pressed tightly against each other as the crowd moves forward.
WARNING: Distressing scenes
WATCH: People climbing away from surging Seoul crowd
South Korea's president Yoon Suk-yeol has announced a period of national mourning over Satuday night's deadly crush that has killed at least 151 people, and injured 82 others in Seoul.
The mourning period will last until the fallout from the disaster is brought under control, Yoon told South Koreans from the presidential office.
"My heart is heavy and I struggle to cope with my grief," he said, adding that he felt "responsible for people's lives and safety".
Officials are now being instructed to carry out an emergency review of all Halloween celebrations and other local festivities.
Following the address, Yoon visited the site of the crush.
Image source, ReutersSouth Korean President Yoon Suk-yeol walks along a street in Itaewon, where people died and were injured in the crush.
Seoul's metropolitan government says it's received 355 missing persons reports since the crush in Itaewon.
The reports are being handed over to police.
In the past hour emergency services have updated the death toll to 151 people, 19 of them are foreigners.
Emergency services are now saying the death toll is 151, including 19 foreigners.
They say that 82 people are known to have been injured.
Officials say the death toll could still rise.
US President Joe Biden has sent a message of support to South Korea over the Seoul Halloween tragedy.
Biden and his wife Jill, the First Lady, sent their "deepest condolences to the families who lost loved ones".
He said: "We grieve with the people of the Republic of Korea and send our best wishes for a quick recovery to all those who were injured.
"The alliance between our two countries has never been more vibrant or more vital – and the ties between our people are stronger than ever.
"The United States stands with the Republic of Korea during this tragic time."
Image source, Getty ImagesPolice at the scene of the deadly crush in Seoul
If you're just joining us, here's a recap of the latest developments:
Canadian Prime Minister Justin Trudeau has sent his "deepest condolences" to South Korea.
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Reports say revellers were killed in the crush when a crowd of as many as 100,000 people were celebrating Halloween. Some of the crowd reportedly surged into a narrow alley in Itaewon, a popular nightlife district in the South Korean capital.
Eyewitnesses have described a scramble to escape a suffocating crowd, as people piled on top of one another. Paramedics, quickly overwhelmed by the number of victims, asked passers-by to help administer first aid.
Others described crowds in the area becoming increasingly chaotic over the evening.
"A number of people fell during a Halloween festival, and we have a large number of casualties," Choi Seong-beom, head of the Yongsan Fire Station, said in a briefing at the scene.
Image source, Getty ImagesEmergency workers helping injured in Seoul
South Korea's President Yoon Suk-yeol has held an emergency meeting and launched an investigation into the cause of the crush.
A task force is being set up to help treat the injured.
The high number of casualties was a result of "many being trampled", fire service officer Choi Seong-beom said.
Read our news story with the latest on the tragedy.
A crush can happen when a large crowd of people start moving in random directions. The chaos of the movement creates an intense force within the jam, trapping those within the crowd and leaving no room for escape.
Reports say the crush in Itaewon began when people fell over in a narrow alley and the crowd surged over the top of them.
But it is not yet clear why the death toll was so high.
Crowds of people who had come to Itaewon for Halloween could be seen leaving the area after the incident.
Emergency services lined the street and tended to the injured.
Revellers made their way out of the tightly-packed nightlife area
Over the past few years, large crowds have surged dangerously on several occasions across the world. The Seoul Halloween crush is among the deadliest.
These are some of the others:
Two 21-year-old men who witnessed the crush say the sight of first aiders treating crowds was like a scene from a war.
WATCH: Seoul crush witnesses describe 'out of control' scene
Su-Min Hwang
BBC Korean Service, Seoul
The number killed has risen to 149 and most were in their teens and 20s, a Seoul fire service officer says.
Choi Seong-beom, chief of Seoul’s Yongsan fire department, told a news briefing that 76 were injured - 19 seriously hurt and 57 lightly.
The dead include two foreign nationals, and 15 foreigners were injured, he said.
Choi added that “finding the cause is the most important”.
France's president has conveyed his "heartfelt thoughts" to the residents of Seoul and South Koreans in a tweet after the crush.
"France is by your side," said Emmanuel Macron.
Ambulances and fire engines line Itaewon district streets
Image source, Getty ImagesDozens of injured people are being treated by medics and first aiders
Image source, Getty ImagesMedics carry injured people into ambulances after the crush
Image source, Getty ImagesCrowds of emergency workers are at the scene in Itaewon
Image source, Getty ImagesAmbulances, fire engines and first aid tents treating injured
Image source, Getty ImagesEmergency workers and police are investigating the cause of the crush
US Secretary of State Antony Blinken has just tweeted this: "We were deeply saddened to learn of the deadly stampede today in Seoul.
"We send our thoughts and deepest condolences to the family and friends of the deceased and injured, as well as to the people of the ROK as they mourn this horrific tragedy."
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Image source, AFPThe crush happened in a part of the South Korean capital that's a thriving spot for young people, filled with bars and restaurants.
Business had picked up after a slump over the Covid-19 pandemic - and one man says it was particularly busy for Halloween.
Park Jung-hoon tells Reuters the crowds were "several ten-folds bigger" than at Christmas or during firework displays.
A witness told Reuters that a makeshift morgue had been set up in a building close to the scene. Dozens of bodies were later carried out on stretchers, the witness said.
President Yoon Suk-yeol is chairing an emergency meeting with senior aides.
"The area is still chaotic so we are still trying to figure out the exact number of people injured," said Moon Hyun-joo, an official at the National Fire Agency.
Foreigners are among those who have been transferred to nearby hospitals. Authorities are investigating the cause of the incident.