West Midlands Police have led a nationwide operation which has seen 12 men arrested in four cities, including Birmingham, over fears they were preparing to launch a terrorist attack.
Unarmed officers from four forces swooped on homes in London, Stoke-on-Trent, Birmingham and Cardiff at 5am. The raids came after several months of surveillance and monitoring by police and MI5 officers.
Sources said the operation was not linked to the recent failed Stockholm suicide bombing or the Yemen al Qaida printer bomb plot.
It is understood the men, all aged between 17 and 28, were held because of fears they were linked to a potential bomb plot inspired by al Qaida.
But one source said an attack was not believed to be imminent and that it was not modelled on the Mumbai commando-style atrocity.
The five men held in Cardiff are all of Bangladeshi origin and community leaders were informed of the arrests.
The operation was led by West Midlands Police, supported by Scotland Yard's Counter Terrorism Command, South Wales Police and Staffordshire Police.
Metropolitan Police Assistant Commissioner John Yates, national lead for counter-terrorism policing, said: "This is a large-scale, pre-planned and intelligence-led operation involving several forces.
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."
Three terraced houses, including two neighbouring properties, were searched in Grove Street, Cobridge, Stoke-on-Trent. A small number of police officers and forensic specialists could be seen in the residential street, a short distance from the city centre.
Police said four men, one aged 19, two aged 26 and one aged 25, were being questioned. One was arrested at a friend's house in Birmingham.
In Cardiff, police arrested three men at their homes in the Riverside area and two at their homes in the Ely area. They were aged between 23 and 28.
In London, police arrested three men aged between 17 and 28 at their homes in the centre of the capital. A fourth home was also being searched.
All 12 suspects were detained on suspicion of the commission, preparation or instigation of an act of terrorism in Britain.
Cardiff councillor Ramesh Patel said police called him several hours after the arrests were made. He said: "It is a shock for me. Cardiff is not one of those places where you would expect this although I realise it can happen anywhere.
"I have to praise police for acting on the information they had very quickly and hopefully these are the right people they have arrested and the problem will be dealt with."
The operation was led by Met Deputy Assistant Commissioner Stuart Osborne, the senior national co-ordinator of terrorist investigations. The arrests were the most high-profile raids in Britain since April 2009, when 12 men were detained in raids across northern England. All were released without charge but authorities still insisted they had thwarted an al Qaida bomb plot based in Manchester.
Iraqi officials claimed last week that captured insurgents believed the Stockholm bombing was part of a series of Christmas season attacks. But both British and German officials insisted there were no specific threats to their countries over the festive period.