A stampede during Halloween festivities in Seoul on Saturday night left 154 people dead and 133 injured after many young people flocked to a popular nightlife district, South Korean authorities said.

The deadly crush occurred in a narrow street in Itaewon, where tens of thousands of people had reportedly gathered for what was the first Halloween revelry since restrictions from the COVID-19 pandemic were eased.

Photo taken in the small hours of Oct. 30, 2022, shows a street crowded with people in Seoul's Itaewon where over 150 people were killed in a stampede during Halloween festivities the previous day. (Kyodo) ==Kyodo

Rescue workers attend to a person in the Itaewon district in Seoul on Oct. 30, 2022. (Kyodo)

A weeklong period of national mourning was declared from Sunday, with authorities trying to get to the bottom of the tragedy.

The deceased, mostly in their 20s, included 20 foreigners, including Chinese, Iranian and Russian nationals, the authorities said.

Among the fatalities were two Japanese nationals, both female, with one in her 20s and another a teenager, according to Japan's Foreign Ministry.

The accident occurred when a large number of people fell over in a downhill alley about 3 meters wide in the area, which was depicted in the popular Netflix drama "Itaewon Class," according to the authorities.

The 2020 South Korean drama series has drawn many Japanese tourists to Itaewon, and there was a remake of the drama in Japan.

But the popular tourist spot turned into a nightmare for many at the weekend as rescue workers tried to resuscitate people lying on the ground.

A total of 848 personnel including 346 firefighters were mobilized to the scene, South Korea's Yonhap News Agency said. The Seoul city government has received reports of around 355 people missing in connection with the crush, the news agency reported.

Some local media cited witnesses as saying that people had flocked to a bar to see a celebrity when the crush occurred.

President Yoon Suk Yeol instructed government officials to ensure the victims receive treatment, and ordered medical officials be deployed in Itaewon and emergency beds secured following what he described as a "tragedy and disaster that should not have happened."

Several world leaders expressed their condolences over the tragedy, with Japanese Prime Minister Fumio Kishida sending a message to Yoon on Sunday saying he is "greatly saddened" and "deeply shocked at the loss of many precious lives."

U.S. President Joe Biden said in a statement, "The United States stands with the Republic of Korea during this tragic time," and assured South Korea that the alliance between their countries "has never been more vibrant or more vital."

Chinese President Xi Jinping also sent a message of condolence to Yoon and urged South Korea to treat the injured and properly handle the aftermath of the incident, according to the official Xinhua News Agency.

According to Yonhap, the crowd crush was the deadliest accident in South Korea since the 2014 sinking of the ferry Sewol that killed more than 300 people, most of them high school students.

Photo shows crowds of people gathered in the Itaewon area of Seoul on Oct. 29, 2022. (Kyodo)

South Korean President Yoon Suk Yeol (C) visits a narrow street in the nightlife district of Seoul's Itaewon on Oct. 30, 2022, where over 150 people were killed in a stampede during Halloween festivities the previous day. (Kyodo) ==Kyodo