Britain's top counter-terrorism officer last night said the arrest of 12 suspects in four cities was “absolutely necessary”.
Scotland Yard Assistant Commissioner John Yates warned searches of homes in London, Cardiff, Stoke and Birmingham could take days to complete as police prepare to question the men.
He said the case highlighted how the public must remain vigilant and urged anyone who witnesses suspicious activity to report it as the national threat level remains severe.
Speaking outside New Scotland Yard, Mr Yates said: “With the information we have, I believe today's arrests were absolutely necessary in order to keep the public safe.”
The 12 suspects, aged between 17 and 28, at least five of whom are of Bangladeshi origin, were arrested during 5am raids at homes in London, Stoke-on-Trent, Birmingham and Cardiff.
Police swooped over fears the group were ramping up their activities after months of surveillance.
The operation was led by West Midlands Police's counter terrorism unit, supported by Scotland Yard's Counter Terrorism Command, South Wales Police, Staffordshire Police and MI5.