Anti-terrorist police officers have raided the headquarters of the radical Islamic group al-Muhajiroun.
Omar Bakri Mohammed's home was searched
Other raids have taken place at the homes of the group's leaders, Omar Bakri Muhammad in Edmonton, north-east London, and Anjem Choudary in Tottenham, north London.
Scotland Yard said search warrants to enter the properties in London and also Essex were executed under the Terrorism Act.
No arrests have been made.
Al-Muhajiroun has been linked to the recent British suicide bombers in Israel and has been behind calls to overthrow the UK government.
A Scotland Yard statement said: "At about 0700 BST officers from the Metropolitan Police Anti-Terrorist branch executed search warrants obtained from Bow Street Magistrates' Court under the Terrorism Act 2000 at a residential premises in North London and another in Essex.
"Shortly afterwards a third search warrant was executed at a business premises in north London. The search is continuing."
Omar Bakri Muhammad told BBC News he was not surprised at the raids.
"It is not the first time we face something similar - the police visit us and take some items and cancel some of our events.
"But the scale of it this time, it seems to me they are really clamping down on al-Muhajiroun.
Box of dynamite
"The police always investigate us, always interrogate us, always check everything we do.
"Everything we do is within the framework of the law which is what you offer to the people living in this country - it surprise me, why now?"
In a statement, al-Muhajiroun warned the British government that it was "sitting on a box of dynamite and have only themselves to blame if after attacking the Islamic movements and the Islamic scholars, it all blows up in their face".
The statement also said: "Early this morning British police and specialist officers, upon the orders of the Blair regime, raided the houses of Sheikh Omar Bakri Muhammad and Mr Anjem Choudary and the movement's offices in north London.
"Al-Muhajiroun are known world-wide for their ideological and political struggle against man-made law and their struggle to establish the Khilafah - the Islamic state.
"They are known not to involve themselves in military activities or recruiting people for operations despite the hype from sections of the media.
"Yet despite this the Blair regime has still seen fit to raid the houses and offices of its leadership."