Anti-terror police arrested 13 people last night in four raids on addresses including some in west London and Hertfordshire.
Swoops were also carried out in Bedfordshire and Lancashire, and the men - all aged between 20 and 40 - are being held at a central London police station.
The arrests come only a day after at least one US bank in the City of London tightened its security following an increase in alert status in major US cities.
The Home Office yesterday said there was no "specific threat" to the UK, but that the terror alert level "remained high".
A Scotland Yard spokesperson said the men were arrested as part of a pre-planned, on-going intelligence-led operation.
"The men have been arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism," the spokesperson said.
Searches continue at several "residential premises". The men are to be interviewed by officers from the Met's Anti-Terrorist Branch.
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