British police on Monday arrested 12 men on suspicion of plotting an act of terrorism, swooping in major pre-dawn raids across the country.
The dozen, aged between 17 and 28, were detained "on suspicion of the commission, preparation or instigation of an act of terrorism in the UK," police said in a statement.
Britain is on high alert after upgrading its perceived terror threat level earlier this year.
"This is a large-scale, pre-planned and intelligence-led operation involving several forces," said Scotland Yard Assistant Commissioner John Yates, who is in charge of counter-terrorism policing.
"The operation is in its early stages so we are unable to go into detail at this time about the suspected offences. However, I believe it was necessary at this time to take action in order to ensure public safety."
The BBC cited sources saying the operation was related to an investigation into Al-Qaeda-inspired terrorism.
Four of the men were from Cardiff, four from Stoke-on-Trent, north of Britain's second city Birmingham in the West Midlands, and three from London.
They were all arrested by unarmed officers at or near their home addresses at around 5:00 am (0500 GMT), apart from one suspect from Stoke who was detained at a house in Birmingham.
They are being held at police stations in central London, northwest England and the West Midlands.
Police said searches were under way at their homes, the Birmingham address and another residence in London.
Home Secretary Theresa May was fully briefed on the raids before they took place, the Home Office interior ministry confirmed.
Britain's current terror threat level is "severe", the second-highest on a five-level grading.
"This means that a terrorist attack is highly likely," the Home Office says.
The threat level was hiked in January after a six-month spell at "substantial" -- the only time it has dipped below the two highest levels since it was set up in 2006 following the London bombings in July 2005.
Those attacks on three Underground trains and a bus killed 52 people, plus the four suicide bombers, who were all British subjects.