London (CNN) -- British counter-terror police arrested 12 men early Monday in a "large-scale, pre-planned, intelligence-led" operation, they said.
The suspects, aged from 17 to 28, were arrested in London, Cardiff in Wales and Stoke in central England. They are due to be questioned Monday, West Midlands police said.
They are all of Pakistani descent, a security source who did not want to be identified told CNN.
Britain's domestic intelligence service, known as MI5, led the operation, the source said.
The suspects were not planning a "Mumbai-style" attack, where gunmen sprayed bullets at crowded public places in India, and there is no known link to a suicide bombing in Sweden earlier this month, according to the source.
The arrests were "necessary" in order to "ensure public safety," Assistant Metropolitan Police Commissioner John Yates said.
He did not say what the suspected target was, but said they were arrested on suspicion of "preparation or instigation of an act of terrorism in the UK".
The counter-terror officers who made the arrests were not armed, police said.
Eleven of the 12 men were arrested at their homes, while the final one was detained at a home in Birmingham. The properties are now being searched.
London saw a large-scale terror attack on July 7, 2005, which left 52 people plus four home-grown suicide bombers dead. A second attempted attack failed two weeks later. CNN
