Nine men, including the radical Islamist preacher Anjem Choudary, have been arrested in London on suspicion of being members of a banned organisation.
Officers are searching 18 premises in London and one in Stoke-on-Trent.
The Met Police said it was part of an ongoing inquiry into Islamist-related terrorism and not in response to any immediate threat to the public.
Mr Choudary is the former UK head of the Islamist group al-Muhajiroun or Islam4UK, banned in the UK in 2010.
The arrested men, aged between 22 and 51, are being held at police stations in central London.
In a statement the Met said the men were being held on suspicion of being members of a proscribed organisation, supporting a proscribed organisation contrary to Section 11 and 12 of the Terrorism Act 2000 and encouraging terrorism, contrary to Section 1 of the Terrorism Act 2006.
Police said a number of residential, business and community premises are being searched; 11 in east London, one in west London, one in north-west London and five in south London.
A residential property is also being searched in Stoke-on-Trent.
The arrests were carried out by the Met's Counter Terrorism Command (SO15) who are holding them for questioning at police stations in central London.