Radical preacher Anjem Choudary, jailed for inviting support for the Islamic State group, has been released.
The cleric was sentenced in 2016 to five and a half years in prison.
He led an extremist network linked to violent jihadists, including one of the killers of soldier Lee Rigby in 2013.
Choudary, 51, has served less than half of his sentence and will complete the rest under strict supervision. Up to 25 measures to control him have been prepared, the BBC understands.
His release from prison comes approximately four months early because of time spent bailed on an electronic tag before his conviction.
Who is Anjem Choudary?
Choudary, from Ilford in east London, once headed up the al-Muhajiroun network - a leading extremist group which was banned under terrorism laws.
The father-of-five did not organise terror attacks, but is considered one of the UK's most prominent radicalisers.
He has been described as a "hardened dangerous terrorist" and someone who has had a "huge influence on Islamist extremism in this country" by former Met Police terror chief Richard Walton.
What happened while he was in prison?
The BBC has learnt from counter-extremism sources that Anjem Choudary refused to take part in deradicalisation courses or exercises while serving the custodial part of his sentence.
He spent most of his time at HMP Frankland, County Durham, where he became the first inmate to be held in a separation unit, designed for the most high-risk terrorism offenders who are capable of radicalising others.
On a number of occasions, Choudary was offered opportunities to speak to mainstream religious leaders and other experts who have successfully turned around the mindset of other extremists.
But on each of those occasions, Choudary refused.
Nevertheless, the prison authorities were not able to delay his release.
Why is he being released now?
His departure from Belmarsh prison came automatically under legislation that allows prisoners to serve the second part of their sentence "on licence" in the community.
This means he will not be free but must comply with a list of conditions. If he breaches them, he risks being recalled to prison.
How will he be monitored?
Police will be closely monitoring Choudary - through probation officers and a requirement that he report to officials.
Choudary is staying at a bail hostel in north London.
He will be in a probation hostel for six months, the BBC understands. The conditions he must obey include:
- A ban from preaching at or attending certain mosques
- He will only be allowed to associate with people who have been approved by the authorities
- He will be allowed one phone and is banned from using an internet-enabled device without permission
- Use of the internet will be supervised
- He cannot travel outside Greater London's M25
- He will not be able to leave the UK without permission.
Earlier this week, it was announced Choudary had his assets frozen and was listed on a global record of known terrorists overseen by the United Nations Security Council.
The asset-freezing order means he will be under extremely strict financial controls which typically mean the authorities will be alerted if he tries to open a bank account or move money.
Prime Minister Theresa May said on Thursday that authorities including the police, prison and probation service had "significant experience in dealing with such offenders".
But John Woodcock, a member of the Home Affairs Select Committee, tweeted that his release was "wrong", "crazy" and "puts the public in danger".
Calling on Home Secretary Sajid Javid to take a tougher counter-terror approach, using Australian law as an example, Mr Woodcock added: "He needs to act fast to protect the public from terrorists being released back onto British streets."
Sir Mark Rowley, the former UK head of counter-terror policing, said it is important "not to overstate his [Choudary's] significance".
"At the end of the day he's a pathetic groomer of others, that's what he has done in the past," said Mr Rowley. "He's not some sort of evil genius we all need to be afraid of."
Analysis: What impact has Choudary's sentencing had?
By BBC home affairs correspondent, Dominic Casciani
When Choudary was charged in 2015 with inviting support for IS, it was a moment of great success for counter-terrorism chiefs - and they were already trying to build cases against other associates.
Some, including close confidantes, were jailed. At least four others, who cannot be named for legal reasons, were subject to a Terrorism Prevention and Investigation Measure (TPim), a form of control that places two years of restrictions on the movements and activities of terrorism suspects who have not been charged with a crime.
Detectives also looked for evidence of standard crimes - such as fraud - as a means to further "disrupt" the network.
The insider view is that this work has been generally successful because it made the targets aware they could no longer act with impunity.
In theory, it created space for the security service MI5 and their police detective colleagues to focus on more urgent threats.