Anjem Choudary is well practised in the art of making contentious or provocative statements. An acolyte of the extremist cleric Omar Bakri Muhammed, who fled the UK for Lebanon, the 47-year-old former lawyer was a founding member of Al-Muhajiroun, which celebrated the 9/11 attacks, and was proscribed along with several other groups that Choudary has fronted, including Islam4UK.

So it's no surprise that when I spoke to him last week he dismissed all allegations of Islamic State (Isis) atrocities, defended the use of crucifixion, and acknowledged Isis leader Abu Bakr al-Baghdadi as "the caliph of all Muslims and the prince of the believers".

Because of his large claims and small following, Choudary has often been derided, not least by fellow Muslims, as a joke figure of no significance. Yet he is known to have had links with a number of people convicted under anti-terrorism laws, as well as the killer of Lee Rigby, Michael Adebolajo.

His views may be unpalatable but, with as many as several hundred British-born Muslims thought to be fighting in Syria and Iraq with Isis and other jihadi groups, they cannot be ignored. Some reports suggest that many of the British jihadis want to return home, having grown disillusioned with the internecine warfare between rebel forces – which prompts the question of what or who inspired them to go in the first place.

Last year a report by the anti-racist organisation Hope Not Hate said that the network of groups run by Choudary has become "the single biggest gateway to terrorism in recent British history" and had "facilitated or encouraged" hundreds of young Muslims across Europe to join the more extreme militants fighting the Assad regime in Syria. While the report noted that there was no evidence that Choudary had instigated any terrorist plots, he was, it said, "a serious player on the international Islamist scene".

In reply, Choudary points out that the security services are well aware of him, and he has never been convicted or charged with a terrorism-related crime.

Whatever people may think of Choudary, it is now clear that a motivated minority of young Muslims share his views. Never was this more chillingly illustrated than in the videos of the beheadings of American journalist James Foley and his fellow hostage, US-Israeli journalist Steven Sotloff, in which a masked young man speaking with a distinct British accent wields a knife and seems to revel in a theatre of sadism.

Choudary insists he doesn't know the knifeman in the video. "I'd recognise his voice if it was someone I knew," he says. But he refuses to condemn the executions or say whether he supports such brutal deeds. "There are circumstances in sharia where there is capital punishment for crimes that have been committed. Now I don't know anything about these journalists, why they were there, whether they were spying or in fact part of the military. Often it turns out that people have other roles as well."

His one certainty in this respect is that responsibility for the murder of Foley and Sotloff lies with the American government. "If you look at the death of James Foley," he says, "you only have to listen to the person who is executing him to know that the blame is the Americans' because of their own foreign policy. The fact is that decades of torture, cruelty and mass murder will have repercussions."

Choudary speaks in the same matter-of-fact way in defence of crucifixion, stoning to death and, indeed, eternal hellfire, which he believes is the fate of everyone who does not accept the Qur'an as the literal word of God. It's a style that has provided him with a cartoonish media image as the go-to Muslim for incendiary declarations.

However, in person Choudary is a surprisingly engaging and humorous conversationalist. His habit of slipping into pious lectures is leavened by a quick wit and a lively appreciation of irony. It's not hard to imagine how he might enthrall impressionable young men with an excitable sense of grievance.

The narrative he repeatedly returns to is one of Anglo-American aggression abroad, domestic suppression of Muslims in the west, the hypocrisy of liberal freedoms, and the inevitable triumph of Islam. It is like listening to far-left rhetoric that is shot through with religious puritanism and a macabre sense of humour.

Thus, for example, he attributes all the deaths in Iraq following the 2003 invasion to the American-led coalition. When I mention that Sunni extremists spouting the same ideology as him were responsible for a considerable slice of the death toll of civilians, he either denies it, changes the subject entirely or says that there was a war on.

His is a black-and-white world in which there was once a peaceful political entity – the Islamic caliphate – that lasted for 1,300 years until it was destroyed by western imperialism. From the time of the Prophet Muhammad in the early seventh century until the end of the Ottoman caliphate in 1924, Choudary maintains, sharia law operated over a wide swath of humanity, bestowing justice and contentment.

On the expansion of Islam itself, he insists that the religion was never "spread by the sword", regardless of historical evidence to the contrary, and that, once educated in Islam, "the people lived peacefully".

Choudary's belief that sharia is the answer to all the world's ills is as genuine as it is zealous. Sharia is misunderstood by non-Muslims, he says, because there is too much focus on hands being chopped off and female adulterers being stoned to death. "But there is also a system of social and economic justice that has to be implemented alongside the penal code," he says.

There is a tendency in public discussion of Islam in Britain either to dismiss Choudary's type of scriptural literalism out of hand or to recast it as a product of alienation, oppression, marginalisation and racism. The idea that anyone sincerely wants to live in a society that exults in horrific executions and religious control of all aspects of public life is one that seems too far-fetched for mainstream society to accept.

Yet the creation in Syria and Iraq of a makeshift state the size of England that is home to several million people has rocked such complacent thinking, particularly as a number of British citizens have taken to social media to promote Isis and appeal to fellow Muslims to fight for the new caliphate.

If Choudary's sermons have inspired some of those Twitter jihadis to go to Syria, it seems he has no plans to join them. The father of four has often been confronted with the fact that he claims state benefits yet hates everything the state that supports him stands for – so why not go to a country better suited to his religious outlook. "Why should I?" has been his stock answer. "I was born here." But is this not an opportunity for a devout Muslim like himself to turn his back on British state-subsidised comforts and answer the call to jihad?

"I don't know how misinformed you are, but if I were even to consider going to Turkey, let alone Syria, not only would I be arrested and my passport confiscated but my wife, my mother and children would be harassed and my accounts frozen. You'll basically be treated as a criminal. We have an apartheid system in this country. Muslims are imprisoned over here. We can't travel abroad."

At other times in our conversation he mentions that he has recently been to Spain and Denmark. Whatever his travel difficulties may be, he admits that he is attracted to Isis.

"From what I understand from people living there, they have security, schools are now being set up where their children are taught about Islam, and they have the basic needs of food, clothing and shelter. They don't see in the public arena things like alcohol, drugs, gambling, these kinds of vices. They've been completely wiped out. I think in many respects it's the kind of society I'd love to live in with my family. Many people I know think the same. That doesn't mean that we're going to train and come back and carry out operations here."

He refers to his British passport as "a travel pass, like an Oyster card" that has no other social or legal significance to him. I suggest that it would be nice if the Isis kidnappers holding and threatening to kill David Haines would see the aid worker's passport in the same way.

"I don't have any information about this fellow over there or what his background is. Before I comment on something like that I have to find out the truth from Muslims."

He rejects the idea that the media or aid workers are needed in Muslim countries and says that it's hypocrisy for the west to protest at unarmed soldiers being lined up and shot dead when the same deeds went on at Guantánamo and Abu Ghraib. I remind him that, however bad the abuses in either place, no one was shot dead, let alone hundreds of people, and that in both cases maltreatment and torture were exposed by the western media he denigrates. But he doesn't want to listen. Instead he claims that "hundreds of thousands of people have been beheaded by drones and daisy cutters".

Most observers would recognise such claims as massive exaggerations. But these are the kinds of inflated figures that are widely accepted by Choudary's young adherents. They would also agree with Choudary when he says that "the British government wants to silence the Muslim voice".

If so, then it has done a poor job in his case. He has been a contributor to just about every TV discussion programme from GMTV to Newsnight. The majority of Muslim commentators, embarrassed by his opinions, would argue that he receives far too much exposure.

We meet in a cafe in Walthamstow in east London, not far from his home. In common with many people, he is shorter than he appears on TV. His beard is long and grey, lending him a premature look of an old sage. Although he is sometimes referred to as a sharia judge, he tells me that he is simply a student of Islam.

In the quiet back room where we speak, a woman is breastfeeding her child. Such a sight would be inconceivable, indeed criminal, in his utopia. Choudary insists that Britain will become a Muslim country operating under sharia law, perhaps by 2050, although previously he has said 2020. He is a relentless evangelist for the cause. At the end of the interview, as he left the cafe, he spoke to a young Turkish Cypriot man who had been listening to our debate, and calmly tried to explain the legitimacy of crucifixion.

Were his dream to come true, and Britain to adopt sharia, would the same right of protest he currently enjoys be granted to those citizens who would oppose such a state?

No, he explains, without a hint of discomfort. "You see we don't believe in the concepts of freedom and democracy. We believe sovereignty belongs to God."

So there would be no right to protest, there would be enforced segregation of the sexes, women would be enshrouded, gay people and apostates killed, no alcohol, all music banned (other than Islamic singing – as long as it was not men to women or vice versa), no theatre or concerts, and the teaching of evolution would be forbidden.

"We have a laugh," he says by way of reassurance, after painting this bleak picture of repressive restrictions. "I could sing an Islamic song to you."

But humour, too, flows only one way. Recently Choudary called the CNN presenter Brian Stelter "shallow" in a debate because the American was offended by Choudary's joking references to 9/11 and 7/7. I ask him if it was shallow to be offended by the Danish cartoons depicting the prophet Muhammad that were the cause of enormous controversy in 2006. At the time, Choudary was one of the outspoken voices in denouncing the cartoons. Placards carried in protests that he helped organise bore slogans like "Massacre those who insult Islam".

"It's not the same," he replies. "Under divine law it is allowed to [joke about] 9/11, whereas insulting the prophet is not allowed."

I ask him if, as someone who mentions the word "justice" a lot, he really believes that stoning a woman to death is the appropriate punishment for adultery.

"For people who have had adultery committed against them, people who have had their wives taken, a lot will say 'I think stoning to death is appropriate'. I was like you, I was completely oblivious to Islam and the Islamic civilisation because I was educated in this system.

"But when you look at the rationale and benefits of it, you realise that it is, in fact, superior."

As a young adult, Choudary was far from an observant Muslim. At Southampton University, where he studied law, he was known as "Andy" and by all accounts was something of a party animal. "I used to get stoned with Andy," recalls one old friend from those days. "He was a really lovely bloke, funny and warm."

The ex-friend says that Choudary showed no interest in religion, but did take exception to the publication of The Satanic Verses. Although that was an event that radicalised a generation of Muslim activists, the former friend suggests it might have been Choudary's failure to land a job with a big legal firm upon graduating that set him off on his path to Salafi righteousness.

Choudary is even less keen to speak about his student years than he to discuss his government benefits. But it is known that meeting Bakri Mohammed, then the leader of Hizb ut-Tahrir, was instrumental in his journey towards what he maintains is the true understanding of Islam.

"I admit that I wasn't always practising," he allows. "But it's not from the tradition of the prophet to expose your sins from the past. I committed many mistakes in my life. You are judged on your latest deed, meaning if you made mistakes and corrected them, people should give credit for that. I thank Allah that he guided me. At the hands of Sheikh Omar Bakri Mohammed, I started to learn more about Islam."

It goes without saying that the vision of Islam espoused by Choudary is not one shared by the majority of Muslims in Britain, but it is one that raises urgent questions about how to accommodate extremist views in a plural society. Choudary believes that there can be no such accommodation.

"The reason why the British government ditched multiculturalism is because Islam is not a spiritual belief," he says. "The sharia has a penal code, it has a foreign policy, it has an economic system, therefore it cannot exist or coexist with capitalism or communism, which are also political beliefs.

"Islam certainly has a spiritual basis which those ideologies don't have, but the reason it does not fit within the model of multiculturalism is because it challenges those things. As a Muslim you cannot live side by side with other beliefs; you continue to strive for sharia."

As long as there are young British Muslims who are inspired by that message, Isis will continue to be seen as the purifying answer to the challenges and complexities of modernity. At least, that is, until they experience the reality.