Fifteen people, including a year-old infant, were injured after fire-bombs were thrown from a nearby three-storey building during the samba street festival in Tokyo on Sunday night.
The police said the casualties suffered light burns on their hands and feet but those injuries were not life-threatening.
After carrying out the attack, the suspect tried to hang himself in his third-floor apartment at about 7.30 pm, but the police rescued him and currently he is in a state of cardiac arrest.
His apartment is situated in the Suginami ward in western Tokyo.
According to the Takaido Police Station, the man, who was in his 60s, threw six bottles of Molotov cocktails that contained flammable liquid which burst into flames once it comes into contact with anything. Some of those were apparently attached to gas cylinders, said police.
The eyewitnesses said the flames reached as a high as two meters off the ground. A firefighting crew managed to extinguish the fires within 90 minutes.
Asahi News reported the neighbours saying that in recent weeks the man had complained that such street festivals were "noisy and disgusting".
"I looked up at the residence and saw the man holding a bottle and a gas burner," Kenji Yoda, who was protecting the samba troop at the end of the parade, told The Japan Times.
"He disappeared soon afterwards. I knew him. He was usually quiet and never caused any trouble," he added.
The police said they will be searching the suspect's residence on Monday.