The attack appeared to be among the worst violent crimes in decades in famously safe Japan.

Details emerged Friday of the horrifying blaze that ripped through a Japanese anime firm, killing dozens including people who tried in vain to reach the roof, but the motive for the suspected arson remained unclear.

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 the dozens of people inside with almost no chance of escape.

That was borne out by the grim toll: 33 dead, with dozens more injured, including several in critical condition.

The attack appeared to be among the worst violent crimes in decades in famously safe Japan.

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.

On Friday people were laid flowers and said prayers at the charred Kyoto Animation building.

"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.

'Like looking at hell'

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... 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 on the scene Friday taking pictures and examining the devastated building, but a motive in the attack remained unclear.

A 41-year-old suspect is in custody, but the investigation has been hampered by the fact that the man suffered serious burns. He has been hospitalised and was reportedly unconscious on Friday.

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

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

The company founded by a husband and wife couple is known for its skilful animations of manga works, and has a reputation as a generous employer in an industry rife with exploitation of artists.

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.

Local restaurant owner Tomoyo Kamada said company employees had often come to her "Cafe Lapin" for lunch.

"They were serious and polite women who gave the impression of loving their work and giving it their heart and soul," she told AFP.

"I still can't believe it. I can't understand why people like them had to suffer such a fate."

'Incredibly dark time'

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."

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 had raised over nearly $1.3 million by Friday afternoon.

"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."