Radical preacher Anjem Choudary has been jailed for five-and-a-half years for inviting support for the so-called Islamic State group.
The 49-year-old was convicted at the Old Bailey after backing the group in an oath of allegiance published online.
Police say Choudary’s followers carried out attacks in the UK and abroad.
The judge, who described Choudary as calculating and dangerous, passed the same sentence on his confidant Mohammed Mizanur Rahman, 33.
Both men were also sentenced to a notification order lasting 15 years, which requires them to tell police if details such as their address change.
Choudary, of Ilford, east London, and Rahman, from Palmers Green, north London, were convicted last month of inviting support for IS – an offence contrary to section 12 of the Terrorism Act 2000 – between 29 June 2014 and 6 March 2015.
The trial heard the pair also used speeches to urge support for IS, which is also known as Daesh, after it declared a caliphate in the summer of 2014.
Counter-terrorism chiefs blame the preacher and the proscribed organisations which he helped to run, such as al-Muhajiroun, for radicalising young men and women including the killers of soldier Lee Rigby in 2013.
But they said they had been unable to act for many years as Choudary – a former solicitor – had stayed “just within the law”.
By Dominic Casciani, BBC home affairs correspondent
As he was sentenced, Choudary’s supporters stood up in the public gallery and shouted: “Allahu Akbar” – Arabic for God is greatist. He smiled and disappeared down to the cells.
For 20 years Choudary has been the police’s headache – now he is the prison service’s. He will start time in the high security unit – a prison within a prison – at HMP Belmarsh in south-east London. Only a few of the most dangerous individuals in the country are ever held there at one time – and the priority will be keeping him apart from the impressionable minds whom Mr Justice Holroyde said he did so much to influence.
Whether the prison service will succeed is unclear. Only last month it published a report that raised serious questions about how well the UK manages violent extremists behind bars. So what happens to Choudary from now on may demonstrate whether jails can securely hold people like him and prevent them from doing further harm.