Anjem Choudary, speaking in April 2015 in a London mosque, has gained a reputation as a Pied Piper for young radicals and a cheerleader for the Islamic State. (Tim Ireland/AP)

Britain’s most infamous defender of Islamist extremism was found guilty of eliciting support for the Islamic State, officials said Tuesday, marking what authorities described as a milestone in the British campaign to combat homegrown terrorism.

The verdict against Anjem Choudary, 49, is the first major conviction against a man seen across Europe as a Pied Piper for young radicals and a cheerleader for the Islamic State. His conviction immediately became the most significant example of how Britain and other European nations are moving to expand their counterterrorism operations, targeting not just active cells but also the voices of incitement.

A top associate of Choudary — Mohammed Rahman, 33 — was also convicted, and both now face up to 10 years of jail time. The verdict came after years in which both men had mostly dodged British justice, doing so by successfully playing the same democratic system they often railed against.

[In Britain, Islamist extremist Anjem Choudary proves elusive]

Choudary is a trained lawyer, and maintained that his polemics — he called for strict Islamic law in Britain and turning Buckingham Palace into a mosque — were expressions of free speech designed to “bait” the British press. His conviction, however, came after he appeared to cross a line by openly supporting the Islamic State.

In lectures and statements posted on social media and YouTube, he encouraged youths to embrace the Islamic State and denied its documented war atrocities, prosecutors said. In one piece of vital evidence, officials said, he pledged allegiance to the Islamic State’s leader in a conversation with a convicted Islamic State recruiter.

“These two men knowingly sought to legitimize a terrorist organization and encouraged others to support it,” Sue Hemming, head of counterterrorism for the Crown Prosecution Service, said in a statement. “They used the power of social media to attempt to influence those who are susceptible to these types of messages, which might include the young or vulnerable.”

The men were convicted on June 28, but the verdict was announced Tuesday, after the conclusion of a related trial. They are set for sentencing on Sept. 6.

Counterterrorism officials and experts have long described Choudary — a soft-spoken orator with a salt-and-pepper beard known for wearing traditional Muslim robes — as a leading figure in the dark networks across Europe that have fostered homegrown extremism and encouraged young Muslims to fight in the Middle East.

“These men have stayed just within the law for many years, but there is no one within the counterterrorism world that has any doubts of the influence that they have had, the hate they have spread and the people that they have encouraged to join terrorist organizations,” Dean Haydon, head of the Metropolitan Police’s Counter Terrorism Command.

[Britain’s anti-terrorism strategy tested by move against prominent preacher]

Choudary, however, has described a crusade against him by authorities.

"If I had even sent one person abroad, really sent them abroad, if they could link that to me, then surely I would have been in court under the anti-terrorism legislation a long time ago," Choudary told Sky News last month. "If you look at my speeches, I have said the same thing for 20 years. For me, it is a matter of worship.”

Yet authorities say that looking at his speeches -- and more -- is precisely what led to his conviction.

During the investigation, Haydon said, 20 years’ worth of material was considered. They included information recovered from 333 electronic devices containing 12.1 terabytes of storage data. In a meeting in a restaurant in July 2014, Choudary and Rahman used Skype and phone texts to contact Mohammed Fachry, a man convicted of recruiting for the Islamic State in Indonesia, and pledged their allegiance to the Islamic State’s leader, Abu Bakr al-Baghdadi. The oath, signed by Choudary, was promptly published by Fachry on an Indonesian website, police said.

Via social and traditional media, Choudary has openly defended known terrorists and called for the spread of Islamic law. His comments often harbored an air of the grandiose, leaving some wondering whether he was a publicity hound more than anything else.

“We believe there will be complete domination of the world by Islam,” he told The Washington Post in 2014. “That may sound like some kind of James Bond movie — you know, Dr. No and world domination and all that. But we believe it.”

But over the past 15 years, counterterrorism experts say, the majority of Britons convicted of offenses related to Islamic extremism have been members or supporters of his shadowy organizations. They included the two men charged with slaughtering Lee Rigby, a British army soldier killed on the streets of London in May 2013.

Yet other than a minor charge for holding an illegal demonstration, a series of allegations against Choudary never really stuck.

In September, 2014, however, exasperated British authorities arrested Choudary and Rahman after mounting their most rigorous case against them of "inviting support" of the Islamic State. Officials focused intensely on their web casts. "The lesson...is that obedience to the caliph is an obligation, if they rule by the sharia," Choudary says in one audio clip -- since taken down from the Internet -- according to the Guardian. "And to obey them obviously means they must be established."

A detailed report compiled on Choudary by Hate Not Hope, a British anti-extremist organization, portrays Choudary as a sinister but savvy figure with deep connections to extremism. Well over 100 Britons with some connection to Choudary and his al-Muhajiroun network have gone to Syria to fight, the group said.

Choudary has disputed that he ever coaxed jihadists to go and fight. But he routinely painted the Islamic State as a religious utopia.

“If you look now in the area controlled by the Islamic State, the Jews, the Muslims and the Christians are living side by side in security,” he once told the Guardian. “It’s not true that people are being slaughtered. Those people who are allied with the previous regime or those who are fighting against the Muslims, certainly they will be fought against.”

Choudary also maintained early and long ties with a host of other radical groups across Europe, including Sharia4Belgium. That group is now labeled a terrorist organization in Belgium, and authorities see it as an incubator of homegrown terror and young jihadists who have joined the Islamic State.

“It seems incredulous that he was allowed to continue all these years,” said Nick Lowles, executive director of Hope Not Hate, a British anti-extremist organization. “This really does put an end to his organization. Others will try to step into his place. But the people who come after him won’t have the same credibility or media profile.”

Karla Adam contributed to this report.

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Today’s coverage from Post correspondents around the world