Tal Cohen/Reuters
File: Muslim preacher Anjem Choudary addressing members of the media during a protest supporting Shari'ah Law in north London October 31, 2009.
Among them was Anjem Choudary, a radical preacher and one of the most high profile Islamists in Britain.
Choudary, 47, was a former spokesperson for the extremist group al-Muhajiroun, which was since been reinvented under multiple names, The Guardian reports.
The men were held on suspicion of belonging to and supporting an organization banned under terrorism laws and encouraging terrorism, police said.
The arrests were not in response to any immediate threat, Scotland yard said in a statement.
Police said the men, aged between 22 and 51, were in custody at stations in central London while 19 properties across the capital and in Stoke-on-Trent in central England were being searched.
Choudary's followers were recently caught handing out pro-ISIS leaflets.
Choudary generates headlines with his colorful damnations of Western governments, and his twitter feed boasting of appearances on NBC and Fox as well as European networks.
His proclamations mix concerns over torture, drone-warfare, and Muslim persecution with conservative Islamism and rhetoric of an epic clash of civilizations. But with no mosque that will have him, Choudary holds lectures and town hall meetings, posting recordings on YouTube.
Britain last month raised its international threat level to the second-highest level of "severe", meaning an attack is considered highly likely, while Prime Minister David Cameron has said the Islamic State militant group in Syria and Iraq poses the country's greatest ever security risk.
(Reuters reporting by Michael Holden; editing by Guy Faulconbridge)
Ben Winsor of BI contributed to this report.