At least 149 people, mostly teenagers and young adults in their 20s, were killed in a crush when a huge crowd celebrating Halloween surged into an alley in a nightlife area of the South Korean capital Seoul on Saturday night, emergency officials said.

A further 65 people were injured in the melee in Seoul's Itaewon district, Choi Sung-beom, head of the Yongsan Fire Station, said in a briefing at the scene.

Nineteen of the injured were in serious condition and receiving emergency treatment, the officials said, adding the death toll could rise.

It was the first Halloween event in Seoul in three years after the country lifted COVID restrictions and social distancing. Many of the partygoers were wearing masks and Halloween costumes.

Some witnesses described the crowd becoming increasingly unruly and agitated as the evening deepened. The incident took place at about 10:20 p.m. (1320 GMT).

"A number of people fell during a Halloween festival, and we have a large number of casualties," Choi said. Many of those killed were near a nightclub.

Many of the victims were women in their twenties, Choi said.

Witnesses described chaotic scenes moments before the stampede, with the police on hand in anticipation of the Halloween event at times having trouble maintaining control of the crowds.

Moon Ju-young, 21, said there were clear signs of trouble in the alleys before the incident.

"It was at least more than 10 times crowded than usual," he said.

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 single narrow alley.

Other footage showed chaotic scenes of fire officials and citizens treating dozens of people who appeared to be unconscious.

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

An unnamed woman who said she was the mother of a survivor said her daughter and others were trapped for more than an hour before being pulled from the crush of people in the alley.

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

Two foreigners were among the dead, and others were transferred to nearby hospitals.

U.S. President Joe Biden and his wife sent their condolences and wrote: "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." read more

British Prime Minister Rishi Sunak tweeted: "All our thoughts are with those currently responding and all South Koreans at this very distressing time."

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.

Authorities said they were investigating the exact cause of the incident.

The disaster is among 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 and prompted widespread soul-searching over safety measures in the country that are likely to be renewed in the wake of Saturday's crush.

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