All seven members of the band — RM, Jin, Suga, J-Hope, Jimin, V and Jung Kook — recently completed South Korea's mandatory military service, and hope to reclaim their status as one of the world's biggest pop acts.
The performance at Gwanghwamun Square on Saturday night launches a global tour spanning dozens of shows across the United States, Europe and Asia, which analysts say could generate hundreds of millions of dollars in revenue.
The hour-long concert comes after the group on Friday released its fifth album, ARIRANG, which sold nearly four million copies in its first day.
"Annyeonghaseyo! We're back," said RM, the band's leader, using the Korean word for "hello", as they opened the concert with the song Body to Body.
Officials expected the BTS concer to draw more than 200,000 people to the Gwanghwamun area, including 22,000 fans who secured free seats in the designated viewing zone and others watching on screens nearby. The show was streamed live on Netflix.
BTS debuted in 2013 and has a legion of global supporters who call themselves the "Army". It became the first K-pop act to top Billboard's Hot 100 chart in 2020 with their first all-English song Dynamite.
Jung Dukhyun, a pop culture commentator, said the impact of BTS' return as a full-group would be tremendous at a time when global fandom for K-pop has grown much stronger, as shown by the success of Netflix's animated sensation KPop Demon Hunters.
Police and city officials imposed stringent crowd-control measures, closing nearby streets, roads and museums, halting the area's subway and bus services, and sealing off dozens of surrounding buildings, in what amounted to a full-day shutdown of the district.
Cars will be barred from the main road between Gwanghwamun and Seoul City Hall for more than 30 hours through to Sunday morning.
The government stepped up anti-terror monitoring, citing global tensions and large crowds of international fans, while police deployed surveillance vehicles and jamming equipment to block unauthorised drones. The restrictions have forced nearby shops to close and deliveries to pause.
Fans and curious onlookers streamed into streets around the square Saturday as thousands of police officers maintained a tight perimeter around the performance venue, channelling the crowds with a maze of fences and buses.
While South Korean officials have taken crowd safety more seriously since a 2022 Halloween surge that killed nearly 160 people, critics say the controls went too far and undermined the symbolism of performing in Gwanghwamun, seen as Seoul's spiritual heart and most prominent gathering space.
The new BTS album, ARIRANG, takes its name from a centuries-old folk song, regarded as an unofficial anthem in both Koreas, whose themes of separation, longing and quiet resilience have echoed across generations.
Gwanghwamun and nearby Gyeongbok Palace provided a sweeping historic backdrop to Saturday's show.
The 14-track record, with lead single SWIM, was recorded in Los Angeles as the group reconvened after years apart.
South Korean officials, including President Lee Jae Myung, expressed hope the event would promote the country's culture and growing soft power.
Some analysts say the group's ARIRANG world tour is likely to become the biggest K-pop tour ever by scale and revenue, with 82 shows planned globally in stadiums of around 50,000 seats.
