Shoko Asahara, the leader of the Aum Shinrikyo cult that carried out a deadly sarin attack on Tokyo's subway in 1995, was executed on Friday, local media reported.
The justice ministry would not immediately confirm that Asahara had been hanged, nor reports that several other cult members would also be put to death Friday.
Asahara and 12 of his followers have been on death row for over a decade for the shocking 1995 chemical attack that killed 13 people and injured thousands more.
His hanging was the first execution in connection with the March 20, 1995 attack which shocked the world and prompted a massive crackdown on the cult.
It paralysed the Japanese capital, turning it into a virtual warzone as injured people staggered out of the underground struggling for breath and with watering eyes.
Some keeled over, foaming at the mouth, with blood streaming from their noses, as the rush hour attack unfolded.
The sarin had been released in liquid form on five subway carriages at different points throughout the network.
The first sign of it was a smell similar to paint thinner, but soon commuters began cough uncontrollably, recalled Sakae Ito, who was on the crowded Hibiya line that day.
"Liquid was spread on the floor in the middle of the carriage, people were convulsing in their seats. One man was leaning against a pole, his shirt open, bodily fluids leaking out."
Police were first alerted just after 8:00am and panic soon set in, with subway workers screaming at people to evacuate and passengers convulsing on carriage floors.
The Japanese Self-Defense Force was called in and descended into the depths in hazmat suits and gas masks to assist the injured and deal with the poison.
Though concerns about the Aum had already been raised, the attack prompted a massive crackdown on the cult's headquarters and the arrest of Asahara and other group members.
He was sentenced to death after a lengthy prosecution during which he regularly delivered rambling and incoherent monologues in English and Japanese.
Born Chizuo Matsumoto in 1955 on the southwestern island of Kyushu, Asahara changed his name in the 1980s, when the Aum cult was being developed.
Virtually blind, he was seen as a charismatic speaker who cloaked himself in mysticism to draw recruits to the doomsday cult he developed in the 1980s.
The Aum cult, now renamed Aleph, officially disowned Asahara in 2000, but it has never been banned and experts say the former guru retained a strong influence, with some members using pictures of him and recordings of his voice for meditation.
Despite the horror that persists over the Aum's subway attack and other crimes, some experts had warned against the execution of Asahara and his acolytes.
They fear his death may trigger the naming of a new cult leader, possibly his second son.
And the execution of Asahara's followers risks elevating them to "martyrs" in the eyes of remaining cult adherents, warned Taro Takimoto, a lawyer for relatives of cultists, in an interview with AFP earlier this year.
"We should have them talk until they die a natural death so that they help prevent a recurrence," he said.