South Korea's President Yoon Suk-yeol declared a period of national mourning on Sunday after a Halloween crush killed some 151 people in a packed nightlife area in Seoul.

Yoon expressed condolences to the victims, mostly teenagers and people in their 20s, and his wishes for a speedy recovery to the injured.

"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."

A huge crowd celebrating in the popular Itaewon district surged into an alley on Saturday night, emergency officials said, adding the death toll could rise, reports Reuters.

Choi Sung-beom, head of the Yongsan Fire Station, said 151 deaths had been confirmed, including 19 foreigners. He told a briefing at the scene 82 people were injured, 19 of them seriously.

It was the first Halloween event in Seoul in three years after the country lifted COVID-19 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 pm.

"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 20s, while the foreigners killed included people from China, Iran, Uzbekistan and Norway, Choi said.

Witnesses described chaos 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 told Reuters.

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.

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

Fire officials and witnesses said people continued to pour into the 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.

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