JOIN USThe suspects - five from Cardiff, four from Stoke-on-Trent and three from London - were detained on suspicion of the commission, preparation or instigation of an act of terrorism and follow several weeks of surveillance.
Police said 11 of the suspects mostly aged between 17 and 28 were detained close to their home at around 5am while the remaining one was arrested from his house in central England city of Birmingham.
It is understood that the plans for the alleged attacks were at an advanced stage.
"The operation is in its early stages and we are unable to go into details at this time about the suspected offenses," a statement issued by the West Midlands police said.
The swoop by anti-terror police comes in the wake of almost a month long alert sounded by major European police forces of an imminent strike by Pakistan based terror groups of a Mumbai-style attack in major cities on the continent.
But, Britain's top anti-terror cop Assistant Commissioner John Yates said the terror plot is not linked to other threats against Europe or the recent bombing in Sweden.
Searches are now being conducted at the home addresses, plus the address in Birmingham and another residence in London, police said.
"The suspects will be held at police stations in central London, the North West and the West Midlands."
The police statement said five of the suspects were from Cardiff, four were from Stoke-on-Trent and three were from London, including the 17-year-old.
The operation is being led by Metropolitan Police Deputy Assistant Commissioner Stuart Osborne, the Senior National Co-ordinator Terrorist Investigations, and is being coordinated from the West Midlands Counter-Terrorism Unit.
It also involves the Metropolitan Police's Counter Terrorism Command, South Wales Police and Staffordshire Police, a spokesman said.
Yates said: "This is a large scale, pre-planned and intelligence-led operation involving several forces.
The arrest were the most high profile anti-terror raids in England since April last year, when 12 men were detained over an alleged plot to bomb a shopping complex in Manchester.