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.
by Dominic Casciani, Home Affairs correspondent
Anjem Choudary is a deeply controversial Islamic figure, a man who many Muslims despise because they believe he causes enormous damage to their position in British society.
Alongside another now-banned cleric, he once spearheaded al-Muhajiroun, a group that argued that the West is fighting a war against Muslims and Islam.
When the government banned the group, some of its former members founded new organisations, including Islam4UK and Muslims Against Crusades - which were also later banned.
The latter group's protests against British soldiers returning from Afghanistan, led to counter-protests and the subsequent formation of the English Defence League.
Mr Choudary has always denied allegations that he has either incited or glorified acts of terrorism.
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).