Radical preacher Anjem Choudary is understood to be one of nine men arrested yesterday 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 were 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 the arrests were not in response to any immediate public safety risk, Scotland Yard said.
Choudary, 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 murder 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 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 per cent of Islamic extremists convicted of terror offences in the UK had current or former links with it.
A resident who lives near a terrace 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 added that 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.”
Anti-extremism campaigners Hope Not Hate 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.”
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.”
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.”
Comment: Page 12; David Cameron puts case for military action: Page 13.