The popular nightlife area of the Itaewon district was the centre of the disaster in South Korea's capital city which has led the country's President Yoon Suk-yeol to call a national period of mouring.

Videos have emerged from the scene showing body bags on the streets, emergency workers performing CPR, and rescuers trying to pull people trapped beneath others.

Although it is not yet clear what caused the incident, there were reportedly around 100,000 people in the area to mark Halloween.

It was the first outdoor Halloween event where no facemasks were required since the beginning of the COVID-19 pandemic.

Prior to the crush, social media users had posted messages documenting the vast crowd and saying that it felt unsafe.

Choi Cheon-sik, an official from the National Fire Agency, said dozens were being treated in hospitals and the death toll could grow.

Officials say it is believed that large amounts of people began pushing forward in a narrow alley near the Hamilton Hotel.

It is said that an emergency broadcast had been sent to every mobile phone in the Yongsan District urging people to return home as soon as possible due to "an emergency accident near Hamilton Hotel in Itaewon".

More than 400 emergency workers and 140 vehicles, including all available personnel in Seoul, were deployed to the scene.

Seoul Mayor Oh Se-hoon is on his way back to the capital city, after cutting short a four-nation tour of Europe as a result of the disaster.

Oh attended the ANOC General Assembly and hosted a farewell dinner for delegates attending the event.

The General Assembly was held in Seoul from October 18 to 21 at the COEX Convention Centre.

Representatives from a total of 204 National Olympic Committees participated in the hybrid meeting, with more than 190 in attendance in-person while the rest joined online.

It was the third time that Seoul had staged the event, after also doing so in 1986 and 2006.

"Our thoughts are with the families and friends of the victims of this terrible catastrophe and with the Korean people," IOC President Thomas Bach said.

ANOC has also expressed its condolences to the families who have lost loved ones.

"Just a few weeks ago, ANOC had the pleasure of hosting its annual General Assembly in the Korean capital, where NOCs received a typically warm welcome that characterises the friendly and hospitable country," an ANOC statement read.

"ANOC has experienced the strength and togetherness of the Korean people, which will help guide them through this traumatic time. 

"The whole NOC family stands together with Korea during this very difficult time."