Radical preacher Anjem Choudary has been arrested on suspicion of being part of a banned terrorist organisation and encouraging terrorism, according to reports.
Choudray is alleged to be part of a group of nine people arrested by Met Police Counter Terrorism Command.
It is believed the men are suspected of being members of Al-Muhajiroun, a British-based Islamic terrorist group who were proscribed in 2010 under the UK Terrorism Act.
The men, aged 21 to 51, were arrested in London and remain in police custody. Police have not confirmed whether Choudary is among the nine people arrested.
A number of residential, business and community premises are being searched as part of the investigation. These include 11 in east London, one in west London, one in north-west London and five in south London. A house in Stoke-on-Trent has also been searched as part of the investigation.
A Met Police spokesperson said: "These arrests and searches are part of an ongoing investigation into Islamist-related terrorism and are not in response to any immediate public safety risk."
Last month, the UK raised its terror threat level from substantial to severe due to the rise of militant groups in the Middle East, such as the Islamic State (also known as Isis).