West Midlands regional police said they had detained five men from Cardiff, four from Stoke-on-Trent and three from London.
“They’ve all been held on suspicion of the commission, preparation, or instigation of an act of terrorism in the UK,” a police spokesman said. The suspects, aged between 17 and 28, were detained by unarmed officers at around 5 a.m. British time, he said.
A senior Iraqi official had said last week that he believed Al Qaeda was planning attacks in the United States, Britain and Europe around Christmas, one year after a failed attempt to bomb a U.S.-bound airliner.
Searches were being conducted at the home addresses of the 12, an address in Birmingham and a separate address in the capital, the police spokesman added.
Metropolitan Police Assistant Commissioner John Yates, who is in charge of counter-terrorism policing nationally, said the swoop was 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,” he said in a statement. “However, I believe it was necessary at this time to take action in order to ensure public safety.”