Anti-terror police were searching homes in four cities as officers questioned 12 men arrested over fears of a looming attack.
Scotland Yard Assistant Commissioner John Yates warned the property searches in London, Cardiff, Stoke and Birmingham could take days to complete as he defended the operation as “absolutely necessary”.
The 12 suspects, aged between 17 and 28 and at least five of Bangladeshi origin, were held during early-morning raids by unarmed officers on Monday.
Police swooped over fears the group were ramping up their activities after months of surveillance and monitoring by counter-terrorism officers and colleagues at MI5.
Sources said they were investigating if the gang was inspired by al Qaida but played down speculation they may have been planning a Christmas or New Year bomb attack.
Mr Yates 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 (yesterday’s) arrests were absolutely necessary in order to keep the public safe.”
Officials said the operation was not linked to the recent failed Stockholm suicide bombing or the Yemen al Qaida printer bomb plot and was not modelled on the Mumbai commando-style atrocity.
The fact that the police officers undertaking the raids were unarmed suggests police had no information that the men had guns, explosive devices or other potential weapons.
The operation was led by the West Midlands Police counter terrorism unit, supported by Scotland Yard’s Counter Terrorism Command, South Wales Police and Staffordshire Police.