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A Japanese man has pleaded guilty to starting a fire at a popular animation studio that killed 36 people.

The Kyoto Animation studio blaze on 18 July 2019 was one of Japan's worst-ever murder cases.

Many of those killed were young animators trapped on the upper floor, and another 32 people were injured.

While prosecutors are likely to seek the death penalty, Shinji Aoba's defence lawyers are seeking an acquittal, citing mental incompetence.

His motives are unclear but reports say Aoba accused Kyoto Animation of stealing his work, which the studio has denied.

In 2019, Japan's then-Prime Minister Shinzo Abe described the attack as "too appalling for words".

Aoba, 45, who nearly died from burns he himself sustained, faces five charges including murder, attempted murder and arson.

He is accused of breaking into the studio, spreading petrol around the ground floor and setting it alight before reportedly shouting "Drop dead".

"I felt I had no other option but to do what I did," said Shinji Aoba, who appeared in the Kyoto District Court in a wheelchair on Tuesday. "I didn't think so many people would die and now I think I went too far."

Aoba's defence team argued that even if he were convicted, he should be given a reduced sentence on account of his "state of diminished capacity", adding that he was delusional at the time of the attack.

Prosecutors told the court Aoba wrongly believed that Kyoto Animation had plagiarised a novel he entered into a contest run by the firm.

But they said he was not controlled by the supposed delusions and can be held fully responsible for the attack.

About 500 people stood in line early Tuesday morning, vying for just a few dozen seats in the courtroom to follow the trial proceedings, Japan's Kyodo News reported.

A verdict is expected next January.

The beloved studio, also known as KyoAni, produces films and graphic novels and is well regarded by fans for the quality of its productions.

Some of its popular animation works include K-On! and The Melancholy of Haruhi Suzumiya.