A man receives medical help at the scene of the stampede in Seoul, South Korea, on Sunday. Reuters

More than 150 people were killed in a stampede at a Halloween event in central Seoul, officials said on Sunday, with South Korea's president vowing a full investigation into one of the country's worst disasters.

The crowd surge and crush hit in the capital's popular Itaewon district, where police estimate as many as 100,000 people — mostly in their teens and 20s — went to celebrate Halloween on Saturday night, clogging the area's narrow alleyways and winding streets.

President Yoon Suk-yeol declared a period of national mourning, telling the country in a televised address that "a tragedy and disaster occurred that should not have happened".

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He said the government "will thoroughly investigate the cause of the incident and make fundamental improvements to ensure the same accident does not occur again in the future".

"My heart is heavy and it is difficult to contain my sorrow," he added, before he visited the scene of the disaster and spoke to emergency workers.

Eyewitnesses described being trapped in a narrow, sloping alleyway, and scrambling to get out of the suffocating crowd as people piled on top of one another.

South Korean national flags fly at half-mast at the government complex in Seoul, South Korea, on Sunday. AP

The fire department said at least 151 people, including 19 foreigners, were killed in the stampede, which occurred around 10:00 pm (1300 GMT).

Most of the victims were young women in their 20s, it said, adding that 89 people were injured.

The Interior Ministry said most victims had now been identified.

"The high number of casualties was the result of many being trampled during the Halloween event," fire official Choi Seong-beom told reporters at the scene, adding that the death toll could climb.

Seoul authorities said they had also received 355 reports of missing people by early Sunday.

Officials said Sunday they had no clear idea of what caused the crush, while eyewitnesses described scenes of chaos as a vast crowd panicked in a narrow alleyway.

Rescue workers and firefighters tend to the injured near the scene in Seoul, South Korea, on Sunday. AP

Local shopkeepers told AFP that the number of people at the annual celebration was "unprecedentedly large" this year -- the first event to be held without Covid-19 restrictions since the pandemic began.

"There were so many people just being pushed around and I got caught in the crowd and I couldn't get out at first too," 30-year-old Jeon Ga-eul told the media.

As questions began to emerge over the lack of security at the event, Interior Minister Lee Sang-min told a briefing that the police force had been occupied on the other side of town.

"I am not certain about the exact number of police personnel deployed (to Itaewon) but a considerable number had been deployed at Gwanghwamun where a large crowd was expected for a protest," he said.

Police had also not expected such a large crowd at the Halloween event, he said.

"The expected size of the crowd in Itaewon did not deviate much from the previous years, so I understand that the personnel were deployed at a similar scale as before."

The bodies of victims, who were killed in the Halloween crush, are covered with sheets in Seoul on Sunday. AFP

Paramedics at the scene, quickly overwhelmed by the number of victims, were asking passers-by to administer first aid.

In an interview with local broadcaster YTN, Lee Beom-suk, a doctor who administered first aid to the victims described scenes of tragedy and chaos.

"So many victims' faces were pale. I could not catch their pulse or breath and many of them had a bloody nose. When I tried CPR, I also pumped blood out of their mouths."

AFP photos showed scores of bodies on the pavement covered by bed sheets, and emergency workers dressed in orange vests loading even more bodies on stretchers into ambulances.

Agence France-Presse