Anjem Choudary, who was jailed for inviting support for Islamic State, was released from Belmarsh prison in south-east London on Friday morning.
The cleric led an extremist network linked to violent jihadists, including one of the killers of soldier Lee Rigby in 2013, and was jailed for five-and-a-half years in 2016.
The 51-year-old has now served half of his sentence and will complete the rest of it under strict supervision, the BBC reported.
If Choudary breaches his conditions of release he will be returned to prison.
Choudary once headed up the al-Muhajiroun network – a leading extremist group which was banned under terrorism laws.
While he did not plan terror attacks, Choudary is still considered “the most dangerous extremist in Britain today”, by anti-extremist charity Hope not Hate.
Speaking ahead of Choudary’s release on Thursday, the charity’s chief executive, Nick Lowes said: “This is a man who has inspired dozens to commit acts of violence and murder. His influence and the damage his actions have caused are unparalleled,” Lowles added.
“His release is likely to breathe life back into the extremist movement he once led.”
Lowles continued: “No other British citizen has had so much influence over so many terrorists as Choudary – we’ve tracked over 120 Islamist terrorists linked to him – and his release is likely to turbo-charge an already-energised far right, acting as a recruiting sergeant for the likes of Stephen Lennon (‘Tommy Robinson’) and violent groups such as the Democratic Football Lads Alliance (DFLA), which could spark unrest.”