Radical cleric Anjem Choudary, long a thorn in the side of British authorities, was jailed in London Tuesday for five-and-a-half years after being convicted of encouraging support for Islamic State (IS) jihadists.

Supporters of the 49-year-old and his co-defendant Mohammed Mizanur Rahman -- who received the same sentence -- shouted "Allahu Akbar" (God is greatest) from the public gallery as the judge announced his decision, according to an AFP journalist.

Judge Timothy Holroyde, in the ruling handed down at the Old Bailey court, said Choudary was "calculating and dangerous" and had shown no remorse.

Dressed in a white robe, Choudary showed no emotion as the sentence was passed.

"A significant proportion of those listening to your words would be impressionable persons looking to you for guidance on how to act," said the judge.

Commander Dean Haydon, Head of the Metropolitan Police's Counter Terrorism Command, said the men "certainly had an influence in radicalising others, poisoning the mind of vulnerable people in the communities.

"They were certainly in contact with terrorists overseas," he added outside court.

A jury had convicted both men in July.

Choudary is the former head in Britain of Islam4UK or al-Muhajiroun, a now-banned group co-founded by Omar Bakri Muhammad that called for Islamic law in Britain.

For two decades, the former lawyer who is of Pakistani descent, stayed on the right side of the law, becoming Britain's most prominent radical preacher.

Among those radicalised by Muhajiroun were the suicide bombers who killed 52 people on London's public transport system in July 2005, and the men who murdered soldier Lee Rigby in the capital in 2013, police say.

- 'Spokesman for extremists' -

The court heard that Choudary had broadcast speeches recognising Abu Bakr al-Baghdadi as the leader of the Islamic State.

It also emerged that Choudary and Rahman pledged their allegiance to ISIS using Mohammed Fachry, a convicted terrorist, to publish the oath that had been signed off by Choudary, on an Indonesian website.

Police chief Haydon said both men had managed to stay "just within the law for many years", a source of frustration for law enforcement agencies.

"We have watched Choudary developing a media career as spokesman for the extremists, saying the most distasteful of comments, but without crossing the criminal threshold," he added.

"Their recent speeches and the oath of allegiance were a turning point for the police -- at last we had the evidence that they had stepped over the line and we could prove they were actively encouraging support of ISIS."

Choudary and Rahman were arrested by officers from the Metropolitan Police's Counter Terrorism Command on September 25, 2014.

Choudary, a father-of-five, previously hit the headlines for organising a pro-Osama bin Laden event in London in 2011.

He also belonged to a group that burned poppies, the symbol of remembrance for deaths in war, during an Armistice Day protest in the British capital in 2010.

In a 2014 interview with AFP, Choudary called on western journalists, civilians and troops in "Muslim countries" to "completely withdraw and allow us to implement the Sharia."