K-pop megastars BTS released their hotly anticipated memoir in South Korea on Sunday, marking their 10th anniversary as a group.

"Beyond the Story: 10-Year Record of BTS" is the septet's first official book, and contains a chronological summary of their musical career as well as hints at their future endeavors, according to their agency BIGHIT MUSIC.

Crowds were kept away by heavy rain on Sunday morning, but some diehard fans braved the weather to gather in front of the Kyobo bookstore in Gwanghwamun, one of the biggest in Seoul.

Sri Lankan fan Lakshi told AFP that if her excitement were measured on a "one to 10 scale, then it is about a million".

Aqilah, a fan from Malaysia, gushed: "I came here since 9 a.m. and I finally got this book!"

The book, which is also being released in the United States, shot to the top of the Amazon and Barnes and Noble bestseller lists in May on the strength of its pre-orders.

It was co-written by South Korean journalist Kang Myeong-seok and members of the band, according to its US publisher Flatiron Books.

The release date of the memoir, July 9, is a nod to a significant date in the septet's history: it is the day the megastars' loyal international fan base, known as ARMY, first came into being 10 years ago.

"(BTS) Thank you for saving me when I needed it," said Audrey, another fan from Malaysia who scheduled a holiday in Seoul to coincide with the memoir's release.

"I wish I could say more, but I am going to cry now," she added.

Over the course of its decade in the limelight, BTS has become a global cultural phenomenon, selling out stadiums and dominating charts around the world while raking in billions for South Korea's economy.

The band is currently on a "hiatus", with members pursuing solo projects while two of the group complete South Korea's mandatory military service.

All able-bodied men in South Korea must serve at least 18 months in the military and, after a years-long debate about whether BTS deserved an exemption, Jin, the oldest member of the group, enlisted last year.

His bandmate J-Hope started his mandatory service in April, South Korean media reported at the time.

Before the hiatus, BTS bagged six No. 1 hits on the Hot 100 and all seven members of the boyband have reached the top songs chart with solo tracks, Billboard said.