Fans of K-pop megastars BTS flocked to hotspots around Seoul Monday to mark the supergroup's 10-year anniversary, with South Korea unveiling a special commemorative stamp series to celebrate.

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 honor 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 traveled to South Korea to celebrate the anniversary, with the Seoul city government running a special program 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."