The arrests have been made in connection with inciting others to commit acts of terrorism overseas and terrorist fundraising, a spokeswoman for the Met said.
All six, understood to include Abu Izzadeen, otherwise known as Omar Brooks, have been taken to a central London police station, she added.
The spokeswoman would not confirm that Abu Izzadeen, who heckled home secretary John Reid at an east London community meeting last year, was among those arrested.
Searches are still ongoing following the raids, which were said to be part of a "long-term proactive and complex investigation into alleged incitement and radicalisation for the purposes of terrorism and alleged provision of finances".
It is not believed that the operation was related to the investigations into the July 7th London bombings in 2005 or the failed attacks two weeks' later.
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