Volkswacht Bodensee - South Korea celebrates 10 years of K-pop megastars BTS

The group's fans -- known collectively as ARMY -- gathered outside the offices of its agency HYBE to take selfies and record TikTok videos in front of a huge mural of the stars being painted on a wall.

Korea Post unveiled a special edition series of stamps in BTS's honour set to go on sale at post offices Tuesday -- the official anniversary of the septet's debut -- having already sold out almost instantly online.

"Obviously BTS are global superstars, but we didn't expect the stamps to be sold out on the day of the online release," a Korea Post official told AFP.

After debuting on June 13, 2013, the group went on to become the first all-South Korean act to dominate the US and UK charts, raking in billions of dollars and building a global fandom in the process.

"Many ARMY BTS fans have asked for the release of BTS stamps, and we also wanted to issue stamps for these global artists for their 10th anniversary," Kim Mi-hwa, Korea Post stamp designer, told AFP.

The band -- currently on a hiatus, with two members performing their mandatory South Korean military service -- released a new digital single last week to celebrate the anniversary.

The single, "Take Two", is purportedly a nod to the band's second chapter after a decade as musicians.

Fans from around the world travelled to South Korea to celebrate the anniversary, with the Seoul city government running a special programme for the visitors.

"It's thrilling to be here. It's thrilling to be surrounded by other ARMY," scientist Anne Micic, 55, a BTS fan from Australia, told AFP.

"I think that's the other thing, that as an ARMY when you meet other ARMY it's almost like you have another family, which is really awesome."

A handwritten letter by J-Hope, who began his military duty in April, was uploaded Monday on Weverse, the worldwide fandom network.

The letter appeared to have been prepared in advance for the anniversary before he joined the military.

"I assume I'd be very military-disciplined" by the time the letter is revealed, he wrote.

"No matter what day it is, I sincerely thank you and love you," he told fans.

- 'BTS saved my life' -

A special map released by the city government for fans visiting from abroad features 13 locations across the South Korean capital.

It includes agency HYBE's office in Yongsan and the historic Gyeongbok Palace, where BTS filmed a special edition of the Tonight Show with Jimmy Fallon.

Fans said it was worth travelling so far to honour their favourite band.

BTS "really saved my life", Claudia Agustin, 23, a fan from Indonesia who works in accounting, told AFP in Seoul, adding that their lyrics had spoken to her during difficult periods of her life.

She said she was "really, really proud" of the group's 10-year anniversary.

"I know how they've been struggling from back in their debut days and then they really make it big. Now everyone really knows them," she added.

Starting Monday, important tourist spots across Seoul, including the Namsan Seoul Tower and Dongdaemun Design Plaza, will be lit up in purple -- ARMY's colour -- in celebration of the anniversary.

BTS members who are not doing their military service yet have been busy with their solo careers -- with Jimin releasing his six-track solo album "Face" in March.

It is likely part of a carefully planned strategy, experts said.

"The most significant part of BTS' 10th anniversary is that they're still here and together as BTS," Jeff Benjamin, Billboard's K-pop columnist, told AFP.

"Every male K-pop group will need to pause or transition due to South Korea's mandatory military service but not every group is adequately prepared as BTS has with preplanned music and content that holds significant meaning.

"Even the new song itself 'Take Two' speaks specifically to this new chapter of BTS with their fans," Benjamin added.

I.Meyer--BTB