SEOUL – Shocked family members collected bodies, parents searched for children and a country sought answers on Sunday after at least 153 people were crushed to death when a crowd in South Korea surged in an alleyway during Halloween festivities.

President Yoon Suk-yeol declared a period of national mourning and designated Seoul's popular Itaewon district a disaster zone after the Saturday night disaster.

"This news came like a bolt from the blue sky," said a father who burst into tears as he collected his daughter's body from a morgue in the nation's capital.

A huge crowd celebrating in Itaewon surged into an alley, killing at least 153 people, most of them in their 20s, emergency officials said, adding the death toll could rise.

The partiers, some still in their teens and many in Halloween costumes, were ready to enjoy the bars, nightclubs and restaurants where the revelry routinely spills over into narrow and often steep side streets.

Instead, the street became filled with people crying for help, while emergency workers desperately sought to free trapped bodies and perform CPR on people splayed across the debris-littered ground.

Choi Sung-beom, head of the Yongsan Fire Station, told a briefing at the scene 82 people were injured, 19 of them seriously. The deaths included 22 foreigners, he said.

Families and friends desperately sought word of loved ones at community centres turned into facilities for missing people.

At least 90 per cent of the victims had been identified by midday, with delays affecting some foreign nationals and teenagers who did not yet have identification cards, the Interior Ministry said.

Makeshift memorials began appearing near the site, with onlookers leaving flowers and notes.

President Yoon expressed condolences to the victims and his wishes for a speedy recovery to the many injured in one of the South Korea's worst disasters and the world's worst stampedes in decades.

"This is truly tragic," he said in a statement, vowing an investigation into the cause of the disaster. "A tragedy and disaster that should not have happened took place in the heart of Seoul last night."

Unruliness, then chaos

South Korean tech and mobile game firms including Kakao and NCSoft pulled their Halloween promotions after the tragedy, while amusement park Everland cancelled Halloween-themed events. Many regional governments and organisations have cancelled or reduced festivals and other celebrations.

The crush of partygoers came as Itaewon, a symbol of freewheeling nightlife in the South Korean capital for decades that was just starting to thrive after more than two years of Covid-19 restrictions, with trendy restaurants and shops replacing seedy establishments.

It was the first Halloween event in Seoul in three years to be virtually free of Covid-19 restrictions and social distancing. Many of the partygoers were wearing masks and Halloween costumes.

24 hours before, there were already warning signs that the festivities were attracting dangerous numbers of people, and victims and their relatives questioned an apparent lack of crowd control.

Early on Sunday costumes and personal belongings mingled with blood spots in the narrow street. Survivors huddled under emergency blankets amid throngs of emergency workers, police, and media.

Many of those killed were near a nightclub, Choi said. The foreigners killed included people from China, Iran, Uzbekistan and Norway, he said.

Witnesses described the crowd becoming increasingly unruly and agitated as the evening deepened. Chaos erupted just before the 10.20pm stampede, with police on hand for the event at times struggling to control the crowds, witnesses said.

Moon Ju-young, 21, said there were clear signs of trouble in the alley before the incident. He told Reuters it was more than 10 times as crowded as usual.

Social media footage showed hundreds of people packed in the narrow, sloped alley crushed and immobile as emergency officials and police tried to pull them free.

Choi, the Yongsan district fire chief, said all the deaths were likely from the crush in the alley.

Makeshift morgue

Fire officials and witnesses said people continued to pour into the alley after it was already packed wall-to-wall, when those at the top of the slope fell, sending people below them toppling over others.

One woman said her daughter, pulled from the crush of people, survived after being trapped for more than an hour.

A makeshift morgue was set up in a building next to the scene. About four dozen bodies were wheeled out on wheeled stretchers and moved to a government facility to identify the victims, according to a Reuters witness.

The Itaewon district is popular with young South Koreans and expatriates alike, its dozens of bars and restaurants packed on Saturday for Halloween after businesses had suffered a sharp decline over three years of the pandemic.

"You would see big crowds at Christmas and fireworks ... but this was several ten-folds bigger than any of that," Park Jung-hoon, 21, told Reuters from the scene.

International leaders offered condolences, including US President Joe Biden and China's Xi Jinping, who noted that Chinese were among the dead and injured.

With the easing of the Covid pandemic, curfews on bars and restaurants and a limit of 10 people for private gatherings were lifted in April. An outdoor mask mandate was dropped in May.

President Yoon held an emergency meeting with senior aides and ordered a task force be set up to secure resources to treat the injured and launch a thorough investigation into the cause of the disaster.

The disaster is the country's deadliest since a 2014 ferry sinking that killed 304 people, mainly high school students.

The sinking of the Sewol, and criticism of the official response, sent shockwaves across South Korea, prompting widespread soul-searching over safety measures in the country that are likely to be renewed in the wake of Saturday's crush.