"It is the darkest day in the history of Japanese animation," a hashtag appeared on Chinese social media Sina Weibo on Thursday evening. As of Friday morning, the hashtag had been circulated over 180 million times.

At least 33 people died and 36 others were injured after a 41-year-old man broke into the Kyoto Animation studio, sprayed petrol and ignited it on Thursday.

The suspect is detained and hospitalized for burns. The reason behind the attack remains unclear.

The fire was extinguished about five hours later. Police and firefighters said 74 people were inside the building when the fire broke out, and most of the 33 dead were found on stairs leading to the three-story building's rooftop exit.

It is Japan's worst mass killing in recent years, which shocked the whole nation as well as people around the world.

Japanese Prime Minister Shinzo Abe wrote on Twitter that the incident was "so dreadful that I am lost for words." He also expressed condolences for the victims.

Kyoto Animation studio is known by fans as KyoAni. Established in 1981, it started to produce animations independently in 2003.

It has earned fame among the world's anime fans by creating high-quality animations, such as "The Melancholy of Haruhi Suzumiya," "K-On!," "Clannad" and "Full Metal Pani." Its production "Violet Evergarden," adapted from a Japanese light novel series, was picked up by Netflix for the global market as well.

Most of the KyoAni animation productions are impressive for their warm stories, subtle and delicate sketches and colors, as well as their deep emotions. They rarely touch upon dark, violent, sexual or controversial themes.

"KyoAni's animations are so tender, and the staff who created them must be the same. I can't understand why they have to suffer this," a comment on Weibo reads.

"The people who have been working days and nights to create dreams for everyone were destroyed; so were their passions," reads another.

Japanese film director Shunji Iwai also took to Weibo to express his anger over the arson suspect: "The place where us creators work has been turned into a target, and people's lives have been treated as nothing. It is unforgivable."

Overseas, fans took to other platforms such as Twitter to express their sadness, mourning, and condolences, as well as what the KyoAni animations meant for them. The hashtag #PrayforKyoAni and #StayStrongKyoAni have been circulated hundreds of millions of times.

On Friday morning, many people arrived at the building and laid flowers.

The tragedy at KyoAni is not only a loss for fans of anime, but for the industry as well.

The animation industry is a pillar of Japanese pop culture, and KyoAni has "an outsized role" in it, according to Yuichi Maeda, Tokyo-based film commentator.

"It's one of the best and largest animation firms in Japan, and with that loss of life, many of the best hands at animation in the nation are likely to be dead," Maeda said. "It's too painful to contemplate."

Maeda added that the KyoAni has "a huge presence in animation" and the loss of the people would be a huge blow to the industry as well.

KyoAni is also one of the few studios which insisted on paying its employees salaries, instead of following the industry’s rule of piecework-based payment. The studio's chief executive Hideaki Hatta is trying to provide a more humane environment for the animation staff, and relieve their pressures, so as to ensure they have enough time and passion for creation.

"I am heartbroken," said Hatta. "It is unbearable that the people who helped carry Japan's animation industry were hurt and lost their lives in this way."