The men, ages 17-28, were being held in jails in Birmingham, Cardiff, London and Stoke-on-Trent on charges of suspicion of the commission, preparation or instigation of an act of terrorism in Britain, The Guardian reported.
The four raids were conducted "to take action in order to ensure public safety," said Britain's top anti-terrorism officer, London Metropolitan Police Assistant Commissioner John Yates.
Police also were searching the four locations where the arrests were made.
Home Secretary Theresa May, briefed on the raids before they occurred, said the "large-scale, pre-planned and intelligence-led operation" involved several forces.
The arrests were coordinated by the West Midlands counter-terrorism unit and involved the Metropolitan Police's counter-terrorism command and police from South Wales and Staffordshire, The Guardian said.
The risk of a terrorist attack in Britain remains "severe" -- the second-highest level -- meaning an incident is seen as highly likely, officials said.
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