The one-hour-long concert, set to be held in the capital's historic Gwanghwamun square on Saturday, marks the release of the seven-member group's first new album in more than three years, and the start of a global tour in April.
BTS made their debut in 2013 and have since gone on to global superstardom, becoming the most streamed Kpop artist globally on Spotify with its members invited to the White House and partnering with the UN General Assembly. In 2022, the group went on hiatus so the members could complete South Korea's mandatory military service.
"The City of Seoul will do its best to make it flexible - to make (the performance) both safe and enjoyable," said Seoul mayor Oh Se-hoon on Thursday as he checked safety measures put in place.
Seoul is staging the event nearly four years after a crowd crush killed Halloween revellers in the city's Itaewon nightlife district.
Memories of the disaster still linger, amplifying pressure on South Korean authorities to ensure the safety of what is expected to be one of its largest-ever public gatherings.
Seoul police have shut streets and erected fences as well as metal detectors around the square, which sits to the south of the Gyeongbokgung Palace.
They have also said they will jam signals of any unauthorised drones. The Seoul government, BTS' management agency HYBE and other organisations involved in the event have also deployed a combined 8,200 personnel to manage the crowds who had already began to gather on Friday.
The Seoul government also said it had set up medical stations and secured 2551 toilets open to the public including in nearby buildings.
Organisers of the concert, which will start at 8pm (10pm AEDT) gave away 22,000 free tickets, but the area is open to non-ticket holders who will be able to watch the performance on Netflix via giant screens.
It will also be streamed live on the platform to 190 countries though journalists have protested strict coverage restrictions by Netflix - including limiting coverage of footage containing audio or video of the performance to a few minutes and restricting video equipment, despite the event being held in a public square - as "excessive".
