K-pop mega-band BTS reunited on Saturday for a free concert in Busan to support South Korea's bid to host the World Expo, four months after the musicians announced they were taking a break from group activities.

Some 52,000 excited fans, many dressed in purple -- the band's official colour -- gathered in the Busan Asiad Main Stadium on Saturday evening, with local reports saying hotels near the venue were fully booked.

The night before the concert, major roads as well as a number of tourist spots in the port city, including the Gwangan Bridge, were lit in purple in honour of the septet.

BTS are credited with generating billions for the South Korean economy, and their label enjoyed a surge in profits despite holding fewer concerts during the coronavirus pandemic.

In June, the septet dropped the bombshell announcement that they were taking a break as a group to pursue solo projects.

Aside from music, the band has championed global progressive causes, from the Black Lives Matter protests to raising awareness on mental health to battling anti-Asian racism in the United States.

"BTS, if you are getting this message, thank you. Thank you so much for everything," an emotional fan from the US told a local broadcaster in Busan on Friday, adding she was inspired by their music when things were difficult.

"I will never give up on myself because you never gave up on me."

'Believe in us'

In June, the band, who are known to be extremely hard-working and maintained a relentless pace of new releases in their nine years atop the charts, said they were "exhausted" as they announced their break.

But just about a month after that announcement, the government appointed them official ambassadors for Busan's bid to host the World Expo in 2030.

The prospect of South Korea's mandatory military service continues to hover over BTS, with its stars ranging in age from 25-year-old Jungkook to Jin, 29, who must sign up by December or risk jail.

"If you believe in us, we'll overcome whatever comes in our way no matter what, and happily perform and make music. Please believe in us," the band's leader RM told fans during the concert.

All able-bodied South Korean men under the age of 30 must perform about two years of military service, mainly because the country remains technically at war with nuclear-armed North Korea.

Seoul's defence minister in August said BTS may be allowed to continue performing and preparing for international concerts even while undertaking military duty.

Jin, who turns 30 in December and faces enlistment, announced during the concert that he would release a solo album soon.

"I was able to collaborate with someone I really adore," he told fans, without explaining further.

Pop-up stores

Even for non-fans, the concert was a stunning event to witness.

"This is the first K-pop concert of my life and I've never seen this many people -- including foreigners from all over the world -- gathered together," Cecilia Yoo, 40, who attended the concert, told AFP.

Aside from the concert, "there were pop-up stores across Busan selling BTS goods, and so many were waiting in line to buy them. Train tickets were sold out. I understood why the band is so important to South Korea's economy," she added.

South Korea is among four countries that have bid to host the World Expo in 2030, along with Ukraine, Italy and Saudi Arabia, according to the expo's organising body.

The Bureau International des Expositions is scheduled to vote on the winning host country next year.

According to the bureau, the historic events -- which take place roughly every five years -- "are a global gathering of nations dedicated to finding solutions to pressing challenges of our time by offering ... engaging and immersive activities".