Twelve men were arrested in Britain this morning on suspicion of plotting a large-scale act of terrorism, police said following pre-dawn raids across the country.

The men, aged between 17 and 28, were detained "on suspicion of the commission, preparation or instigation of an act of terrorism in the UK," said West Midlands Police, which serves the second city of Birmingham.

"This is a large-scale, pre-planned and intelligence-led operation involving several forces," said Assistant Commissioner John Yates, the officer in charge of counterterrorism 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."

Four of the men were from Cardiff, four from Stoke-on-Trent, north of Birmingham, 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.

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 interior ministry says.