Anjem Choudary has become one of Britain's most outspoken defenders of the Islamic State. Critics say he's helped indoctrinate dozens of impressionable young men who have gone on to plot or carry out attacks. (Griff Witte, Karla Adam and Casey Capachi/The Washington Post)

— As British war planes arc through Middle Eastern skies and security services race to unravel terrorist plots at home, the nation’s most prominent propagandist for the Islamic State sits in a London sweets shop, laying out his radical vision between bites of dessert.

Iraq and Syria, Anjem Choudary says confidently, are only the beginning. The Islamic State’s signature black flag will fly over 10 Downing Street, not to mention the White House. And it won’t happen peacefully, but only after a great battle that is now underway.

“We believe there will be complete domination of the world by Islam,” says the 47-year-old, calmly sipping tea and looking none the worse for having been swept up in a police raid just days earlier. “That may sound like some kind of James Bond movie — you know, Dr. No and world domination and all that. But we believe it.”

With such grandiose proclamations, it is tempting to dismiss Choudary as a cartoonish hate preacher straight out of central casting. Many do. But harder to ignore is his record of inspiring impressionable young men to carry out violence in the name of Islam — both in Britain and overseas.

Counterterrorism officials and experts say Choudary and the many shadowy groups he has fronted have directly contributed to the indoctrination of dozens of people who have gone on to plan or commit attacks in the United Kingdom. His network, they say, has also become a vital facilitator in the flow of some of the thousands of Europeans who have swarmed to the battlefields of Iraq and Syria, and who could return to carry out attacks in the West.

Map: Flow of foreign fighters to Syria

But even as a coalition that includes Britain and the United States wages war on the Islamic State, Choudary and other enablers remain free to spread their seductively messianic ideology on the streets of the United Kingdom and globally, through the Internet. They do so by taking advantage of the very rights they condemn as un-Islamic and by using their considerable charisma to lure lost souls.

“These guys are very good at knowing where the limits of the law lie,” said Richard Barrett, a former counterterrorism director with Britain’s foreign intelligence service, MI6. “They’re also very slick, very plausible and very persuasive.”

Their elusiveness helps explain why extremism continues to flourish in Britain despite more than a decade of concerted effort to stamp it out, and why security officials remain so nervous about what Prime Minister David Cameron has called the greatest terror threat the country has ever known.

Britain has long been a locus of Islamist extremism, with its large Muslim immigrant communities and its tolerant approach toward those with radical views. In the late 1990s and in the years after the Sept. 11, 2001, attacks, north London’s Finsbury Park mosque became a critical way station for global terrorists.

But years of aggressive policing and intelligence efforts have shifted the extremist threat away from Britain’s mosques and into the hands of freelancers who are much harder to monitor and control.

Dancing around the law

Choudary — a lawyer by training, not a preacher or religious scholar — has proved particularly adept at staying out of reach of the authorities.

Late last month, police raided his home on the suspicion that he was involved in terrorism-related activities, and his passport, phone and laptop were confiscated. But authorities held him for only a night before letting him go.

Admirers of Anjem Choudary are photographed with him in a shop in Ilford, northeast London, on Oct. 1. (Andrew Testa/For The Washington Post)

Choudary has been, for nearly two decades, at the forefront of a succession of groups — including al-Muhajiroun, Islam4UK and Muslims Against Crusades — that have been outlawed for extremist activities. Once a group was banned, Choudary quickly set up a new one with a similar structure and many of the same members but with a new name.

The majority of Britons convicted of offenses related to Islamic extremism in the past 15 years have been members or supporters of Choudary’s network. Choudary himself, despite multiple arrests, has never been convicted of anything more than staging an illegal demonstration.

Days after his latest release, sitting in the sweets shop in the northeast London neighborhood of Ilford, he is unbowed and almost dares the government to come after him.

“You need sufficient evidence,” he says, as numerous admirers stop by to vow their support. “And they have no evidence whatsoever.”

Choudary, whose parents emigrated from Pakistan but who was born and raised in Britain, has a thick black beard that is turning as white as his spotless shalwar kameez — the traditional South Asian garment that is ubiquitous in Ilford, where Choudary lives. He wears rimless spectacles, speaks softly, and smiles often, even when delivering a bloodcurdling message.

Choudary maintains that he has never directly encouraged young people to fight for the Islamic State but acknowledges his followers have a habit of “popping up” in Syria.

What they do there he does not know, Choudary claims. He says he doubts they are there to fight because the Islamic State already has more than enough recruits.

“There are up to 1,000 people wanting to join the Islamic army every day,” he says approvingly.

As for the Islamic State’s execution of Americans, Britons and countless Syrians and Iraqis, Choudary insists that the claims are overstated and that those the organization has killed deserved to die.

It’s that sort of dance — lauding a terrorist group, without actually inciting violence — that has kept Choudary out of prison.

The same cannot be said for his followers. According to the anti-extremism group Hope Not Hate, at least 80 people with ties to Choudary or his organizations have been implicated in terrorism cases ranging from the July 2005 bombings on the London transit system to last year’s killing and near-decapitation of a 25-year-old British soldier, Lee Rigby, on a London street.

“If you look at the people who have been through Choudary’s organizations, it’s deeply worrying. And his role has been pivotal,” said Nick Lowles, Hope Not Hate’s chief executive. “There’s no evidence that he’s directly implicated in these plots. But he gives the ideological justification for jihad, for war against the West. And around him are figures who are much more involved in sending people to Syria or encouraging people to go to terrorist training.”

Under Choudary’s influence

Richard Dart is among those who found his way into Choudary’s orbit — and nearly became a killer.

Dart, the son of schoolteachers, was raised middle-class in the quaint English town of Weymouth. But he fell under Choudary’s spell while in his late 20s and soon converted from Christianity to radical Islam.

“People like Anjem know what it is that people like Richard are craving — identity, respect, empowerment,” said Dart’s stepbrother, Robb Leech. “They push all the right buttons — make them feel special. And once you’re in the door, it’s like family. They all look after each other.”

Dart and other Choudary followers burned an American flag in front of the U.S. Embassy on the anniversary of 9/11 and cursed British troops during their homecomings.

But Dart wanted to go further. He traveled to Pakistan to seek terrorist training. Upon his return, he talked with a fellow extremist about a possible attack in Royal Wootton Bassett, the British town through which soldiers’ remains have traditionally been repatriated, or a return to Pakistan to link up with the Taliban. Police arrested Dart before he could follow through on his plans, and in April 2013, he was sentenced to six years in prison.

Leech said he doesn’t think Choudary directed Dart to carry out violence. But neither does Leech think his stepbrother would be in prison today without Choudary’s influence.

“Anjem was Richard’s role model — completely,” said Leech, who has made two documentaries about his step-brother’s conversion, arrest and conviction. “If Richard wanted to know something, he’d ask Anjem.”

Choudary’s ease in converting lost souls may come because he once was one. Known as “Andy” during his university days, Choudary was reputed to be a partier who indulged in drinking, drugs and women, according to multiple accounts.

It was only after meeting Omar Bakri Mohammed, a radical preacher who has since fled into exile, that Choudary remade himself as Mohammed’s pious lieutenant and, later, the leader of his own hundreds-strong organization.

Choudary’s network isn’t limited to Britain. He has encouraged acolytes across Europe to set up groups similar to his own, providing them with inspiration and guidance.

Last month, 46 members of one such group, Sharia4Belgium, went on trial in Antwerp in Belgium’s largest Islamic extremism case to date. Only eight defendants were present in court, with the rest presumed to be in Syria, either dead or still fighting.

After Choudary visited Indonesia last October and spoke at a Sharia4TheWorld rally, the Islamic State saw a surge in Indonesian volunteers. Choudary also has a prodigious following on the Internet, where he keeps a steady stream of vitriol churning through Twitter.

It’s doubtful that Choudary himself is sending would-be fighters to Syria, said Raffaello Pantucci, a counterterrorism expert with the Royal United Services Institute think tank. “He’s so careful about what he says and does,” Pantucci said.

But Hope Not Hate estimates that several hundred among the several thousand Europeans who have gone to fight in Syria passed through Choudary’s organization or one of its affiliates — making it the continent’s largest recruitment network for Islamist militants.

The relative ease of traveling to Syria from Europe is one reason it has become such a popular destination for would-be militants. Still, facilitators play a vital role, recruiting would-be fighters and making sure they have the necessary connections, instructions and money to link up with militant groups on the ground, said Gilles de Kerchove, the European Union’s counterterrorism coordinator.

Despite numerous arrests of former and would-be fighters, very few facilitators have been prosecuted.

Choudary has so bedeviled British security officials that the government recently proposed changing the law in a way that seems tailor-made for him: Authorities would be able to restrict individuals’ travel and activities, even without a criminal conviction. The changes could also keep people like Choudary off the air and off the Internet — in much the way Sinn Fein leader Gerry Adams was barred from television and radio during the height of the violence in Northern Ireland.

But the strategy has drawbacks.

“The danger is that we make them into martyrs,” said David Hanson, a former British counterterrorism minister.

The longer-term solution is not necessarily to change the law but instead to undermine Choudary’s way of thinking, said Ghaffar Hussain, a former Islamist who now leads the anti-extremist Quilliam Foundation in London.

“There’s a lot of bluster coming from these guys,” Hussain said. “You need to stand up, challenge them and rubbish their ideas.”

That’s already begun. A Facebook page called Muslims Against Anjem Choudary has proved popular, and at some mosques, he’s denounced by the clerics.

Leaders of the East London Mosque have even called the police on Choudary’s followers after they carried out vigilante “Muslim Patrols” — harassing women for not covering up and knocking beer out of people’s hands.

“Sharia law says you can’t impose it on people who don’t want it,” said Salman Farsi, a mosque spokesman. “We are dealing with very simplistic individuals. It’s sad, because they don’t understand the religion.”

But for Choudary, the critics hardly matter as long as there’s an ample supply of followers.

And there seems to be no shortage.

Sitting in the sweets shop in Ilford, recounting details of his recent arrest and release, Choudary is interrupted by a young man with a wispy beard, pushing a baby carriage.

“I saw you on the news yesterday, on the BBC, standing on Ilford Lane,” the young man tells Choudary earnestly. “I support a lot of what you — well, everything that you stand for. I think it’s the media that is against us. And obviously, the establishment.”

“Of course,” Choudary says, nodding and offering his phone number. “Of course.”

The young man leaves with a smile, and a promise to call.

Karla Adam in London and Greg Miller in Washington contributed to this report.