Mourners gathered a the site of Japan's worst mass killing in 18 years, laying flowers and offering prayers for the 33 people killed in an arson attack on an animation studio.

Details have emerged of the terrifying inferno that tore through the Kyoto animation firm.

The public broadcaster NHK is reporting that Japanese police have identified the man suspected to have started the fire as Shinji Aoba. 

Eyewitnesses described a fire that was like "looking at hell," after a man apparently doused the production company office in the city of Kyoto with flammable liquid and set it alight.

Experts and firefighters said the blaze would have raged uncontrollably throughout the structure almost instantly, leaving dozens of people inside with almost no chance of escape.

It sparked an outpouring of grief in Japan and beyond, as fans of the country's famed anime industry expressed horror at the enormous loss.

"These young people were the age of my grandchildren," 78-year-old Sachiko Konishi told AFP of those killed in the attack, most of whom are presumed to be young employees of the company.

"If my grandchildren died under circumstances like this, I wouldn't want to go on living."

Yasuko Tomita, 59, was offering prayers near the building, which was still cordoned off.

"I'm praying to help the souls of those killed find release," she said.

Eyewitnesses described a raging inferno that left those outside unable to even approach to help people trying to escape.

"There was a person who jumped from the second floor. The person jumped, desperately trying to get away from the building but we couldn't rush to help because the fire was so strong," one local woman told an online affiliate of the Asahi Shimbun daily.

"People with severe burns were crying uncontrollably, just completely dazed," she added.

"It was like I was looking at hell."

Police investigators were at the scene taking pictures and examining the devastated building.

No motive has yet emerged in the apparent attack.

Police have detained a 41-year-old suspect, but their investigation has been hampered by the fact that he suffered serious burns in the blaze.

He has been hospitalised and is reportedly unconscious.

Only sketchy details have emerged about the man, who reportedly shouted "drop dead" before setting the fire.

Local media said he did not appear to have any ties to Kyoto Animation. He had reportedly also brought knives and a hammer to the building.

Some reports suggested the man believed the company had stolen his work, with the local Kyoto Shimbun newspaper reporting that he told police: "I set the fire because they stole novels."

Yesterday, Kyoto Animation president Hideaki Hatta said the firm had received emailed death threats in the past but gave no further details.

There was still little information on the victims of the blaze. Local police said 12 men and 20 women were among the dead, with one victim not yet identified.

Many of the bodies were found on a stairwell leading to the roof, suggesting people were trying to escape the flames when they were overcome.

A local fire department official told AFP the building was in compliance with fire safety rules.

But he said a gasoline-fuelled fire would have burned too quickly for people to outrun it, with a spiral staircase connecting the floors of the building likely helping the blaze spread faster.

"If a large quantity of gasoline was poured, it must have turned into gas very quickly, causing explosions," he told AFP.

"Smoke and flames would have risen up instantaneously."

Keizo Harafuji, a former scientific investigation officer for the Tokyo police, also said those inside would have had little chance of escape.

"Flames spread incomparably faster with gasoline than in a normal fire," he told public broadcaster NHK.

"The temperature in a room can rise to 1,500 degrees in some cases. Iron can melt in this heat and people cannot breathe."

The blaze hit hard in Japan's anime industry, one of the country's best known cultural products. An online fundraiser organised by an American anime licencing firm has raised over $1m.

"Thank you to everyone who has shown their support for those impacted by the tragedy at KyoAni," the organisers Sentai Filmworks wrote, using a nickname for Kyoto Animation.

"Together, we can bring light to this incredibly dark time."