Homes and businesses across England are being searched and 24 people questioned after police say a plot to blow up planes from the UK to US was disrupted.
They say they are convinced they have the key players in custody, but a wider investigation is only just beginning.
Peter Clarke, the head of Scotland Yard’s anti-terrorist branch, said the network involved was large and global.
And US Homeland Security Secretary Michael Chertoff said the plot was “in some respects suggestive of al-Qaeda”.
British police said it could have caused “mass murder on an unimaginable scale”.
Sources in the UK have confirmed that they believe an attack may have been imminent – possibly in the next few days.
Police probe flights terror plot