Seoul: At least 151 people were killed in a stampede when a huge crowd celebrating Halloween crammed into an alley in a night life area of the South Korean capital Seoul on Saturday night, emergency officials said.

A further 150 people were injured in the crush in Seoul's Itaewon district, Choi Sung-beom, head of the Yongsan Fire Station, said in a briefing at the scene.

South Korea's President Yoon Suk-yeol declared a period of national mourning on Sunday after a Halloween crush and expressed condolences to the victims' families.

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

Many of the injured were in serious condition and receiving emergency treatment, the officials said.

The incident took place at about 10:20 p.m. (1320 GMT). A large number of people fell down in the alley during the festivities, Choi said. Many of the party-goers were wearing masks and Halloween costumes.

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.

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 to free.

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

A Reuters witness said a make-shift 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.

It was the first Halloween event in three years after the country lifted COVID restrictions and social distancing. Some witnesses described the crowd becoming increasingly unruly and agitated as the evening deepened.

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.

Curfew 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 Suk-yeol presided over 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 were among those transferred to nearby hospitals.

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