The self-styled Muslim cleric was born in Iran and sought political asylum in Australia in 1996.
Man Haron Monis is well known to the Australian police and is currently on bail for being an accessory to the murder of his ex-wife.
He is also facing more than 40 s**ual and indecent assault charges.
These relate to time allegedly spent as a self-proclaimed “spiritual healer” who dealt with so-called black magic at a premises in western Sydney, Sydney Morning Herald reported.
Man Haron Monis has previously been convicted of sending offensive letters to the families of deceased Australian soldiers.
There are reports that one of his demands is that an Islamic State flag be delivered to the Lindt cafe in Martin Place.
However, Man Haron Monis has no known links to jihadist groups, and commentators have suggested he is likely to be working alone.
Iranian-born Man Haron Monis, also known as Sheikh Haron and Mohammad Hassan Manteghi, wrote on his website that he used to be Shia but is no longer.
He said he “used to be a Rafidi, but not any more”, using a pejorative term that some hard-line Sunni Muslims use to refer to Shias.
Man Haron Monis denied the criminal charges against him, saying they were politically motivated. He compared the accusations of sexual assault against him to the case of Julian Assange, founder of the whistle-blowing website WikiLeaks.
His website had been suspended by the time his identity was revealed in the media in connection with the siege.
Man Haron Monis’ former lawyer, Manny Conditsis, told ABC News that Monis was an isolated figure.
“His ideology is just so strong and so powerful that it clouds his vision for common sense and objectiveness,” he said.
Manny Conditsis, who represented Man Haron Monis in 2013 when he was charged with being accessory to the murder of ex-wife Noleen Hayson Pal, said he thought the suspect might feel “he’s got nothing to lose”.
“Hence participating in something as desperate and outrageous as this,” he said.
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