Radical preacher Anjem Choudary is understood to be one of nine men arrested today as part of an investigation into Islamist terrorism.
The men, who were all seized in London, were arrested on suspicion of being members of, or supporting, a banned organisation, the Metropolitan Police said.
Al-Muhajiroun is understood to be the banned organisation in question, sources told the Press Association.
Counter-terrorism police are searching 18 addresses across London and one in Stoke-on-Trent.
The men, who are aged between 22 and 51, were arrested as "part of an ongoing investigation into Islamist-related terrorism and are not in response to any immediate public safety risk", Scotland Yard said.
Earlier this week, it was reported that Choudary said he has no sympathy for Alan Henning, a volunteer aid worker who was captured in Syria.
Islamic State (IS) militants threatened to behead the 47-year-old in a video released earlier this month, which showed the murder of another British man, David Haines.
Choudary is reported to have said: "In the Qaran it is not allowed for you to feel sorry for non Muslims. I don't feel sorry for him."
The radical preacher, who co-founded the now banned group al-Muhajiroun, has had contact with a number of worshippers who have later gone on to be convicted of terrorism.
Fanatics Michael Adebolajo and Michael Adebowale, who were both jailed earlier this year for the brutal slaying of Fusilier Lee Rigby, were both seen at demonstrations organised by al-Muhajiroun.
Choudary said he knew Adebolajo, who was pictured beside him at a rally in 2007, and the second founder of the group, Omar Bakri Mohammed, claimed that he had spoken to the future killer at meetings.
The group, which has changed names a number of times, was banned in the UK in 2010, and a study suggested that in the preceding 12 years 18% of Islamic extremists convicted of terror offences in the UK had current or former links with it.
Choudary, whose outrageous demonstrations and pronouncements often earn him more than his due in the media, has faced accusations of inciting hatred in the past.
Anti-fascist campaigners Hope Not Hate alleged in a report ‘Gateway To Terror’, that Choudary and al-Muhajiroun lead a network of hardline Islamist organisations across Europe from Belgium and the Netherlands, to France, Denmark and Italy to Germany, Norway and Finland, the largest extreme Islamist network in Europe.
Choudary's network of influencers has sent hundreds of British Muslim citizens to fight in war zones, including at least 50 to Syria, and several hundred to Afghanistan and Pakistan, the report claims.
The preacher, who the report describes as being "energised by public outrage", has lead bullish campaigns, burning poppies, demonstrating at soldiers' funerals, defending the atrocities of 9/11 and the London tube bombings. A Sunni Muslim, he has also led sectarian demonstrations, targeted at Shia Muslims.
The group controversially called for the creation of independent Muslim emirates within the UK cities where Sharia Law could be imposed. The three proposed locations were Bradford and Dewsbury in Yorkshire and Tower Hamlets in East London. His group has appeared in many guises, with most incarnations banned by the Home Secretary, including Islam4UK and Muslims Against Crusades.
Hope Not Hate today welcomed the arrests. The group's chief executive Nick Lowles said: "For over a year, since our own extensive investigations into Anjem Choudary and his disciples, we've been saying that more must be done to curb this hate-supporting and recruiting organisation."
A resident who lives near a terraced house where Choudary was believed to have lived in Walthamstow, east London, said the preacher and his family moved out about a week ago.
She said Choudary had lived there for around two years and claimed there was police activity at the weekend after he left.
The woman, who did not wish to be named, said: "There were police cars and officers walking around. I can't say whether they actually went in."
Scaffolding was in place at the house in Walthamstow as work was being carried out.
Derek Rayner, a retired painter and decorator who has lived on the street for 50 years, said of Choudary: "I was very much aware he was living in the street.
"I didn't know much about him, other than what I read in the papers.
"He kept himself to himself. It was noticeable that there were comings and goings. Sometimes you wouldn't see him for a couple of weeks. Then when he was back, you would see him walking up and down the road."
He said he stopped speaking to Choudary following comments he reportedly made about murdered soldier Lee Rigby and Islamic State militants.
"I used to speak to him to just say good morning but having seen the things that he had been saying... I won't give him the time of day now."
He said he had not seen the cleric for around a week.
"I wasn't totally sure he had moved out - I thought he might have just moved out while they were doing the house up. They have totally gutted the house.
"I have seen him moving stuff out. I wasn't sure if it was going to be permanent or just temporary. "
He said he had not seen any police activity but added: "I have been informed there was a lot of police presence here over the last week. My neighbour said there has been police cars coming up and down, policemen knocking on his door apparently."