Six men have been arrested on suspicion of incitement offences by anti-terror officers from the Metropolitan Police.
They include Abu Izzadeen, also known as Omar Brooks, who made headlines when he heckled Home Secretary John Reid at an event last year.
Five arrests took place in London and the sixth in Luton, Bedfordshire.
Scotland Yard said the arrests related to allegations of inciting others to commit acts of terrorism abroad and fund-raising for terrorists.
The BBC understands that the arrests are connected to an alleged incident at a mosque in 2004. The men were taken to an unnamed police station in central London.
The BBC understands that among the six arrested is Rajib Khan, a 28-year-old man from Luton.
The other men arrested are a 24-year-old, 35-year-old and 21-year-old, all from parts of East London, and a 27-year-old from Southall in the west of the capital.
Mr Izzadeen, a former electrician, lives in east London and is married with three children.
Scotland Yard added that searches were continuing in connection with the raids.
A Metropolitan Police spokesman said: "The arrests form part of a long-term pro-active and complex investigation into alleged incitement and radicalisation for the purposes of terrorism, as well as alleged provision of financial support for international terrorism."
