London (CNN) -- British counter-terror police arrested 12 men early Monday in a "large-scale, preplanned, intelligence-led" operation, they said.
The suspects, ages 17 to 28, were arrested in London, Cardiff in Wales, and Stoke and Birmingham in central England. They were due to be questioned Monday, West Midlands police said.
A security source, who did not want to be identified, told CNN initially that the suspects were believed to be of Pakistani descent. The source later said that some of them may also be of Bangladeshi descent. Officials are still trying to confirm the identities of the suspects, the source added.
Britain's domestic intelligence service, known as MI5, led the operation, the source said.
The suspects were not planning a "Mumbai-style" attack, in which 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.
A senior UK official not authorized to speak on the record said the suspects were more "aspirational" rather than equipped and "ready to go."
The arrests were "necessary" to "ensure public safety," Assistant Metropolitan Police Commissioner John Yates said.
He did not say what the suspected target was, but said the suspects 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 were being searched.
London saw a large-scale terror attack on July 7, 2005, that left 52 people plus four home-grown suicide bombers dead. A second attempted attack failed two weeks later.
CNN's Dan Rivers contributed to this report.