British counter-terrorism police carrying out pre-dawn raids in the city of Birmingham arrested eight men accused of planning a kidnapping, police said.
The men were accused of committing, preparing or instigating terrorism, police said.
Government officials, speaking on condition of anonymity because they were not authorized to speak about the operation, said the allegation marked a new chapter in terrorist activity in Britain.
Counterterrorism officials said police believed a plot was being formed to kidnap an unidentified target. It was unclear whether the target was to have been a prominent person. The officials spoke anonymously because of the sensitivity of the investigation.
Police would not comment on Sky News TV reports that part of the plan was to behead a hostage and post the act on the Internet, and neither the Home Office nor West Midlands police could confirm details of the alleged plot.
Officers raided several homes in a major operation in a predominantly Pakistani neighborhood in the city in central England. Two Islamic bookshops were cordoned off.
The arrest operation was led by the Midlands Counterterrorism unit, supported by West Midlands Police and London's Metropolitan Police.
Last week, police arrested five people in terrorism investigations in the English cities of Manchester and Halifax.
Rizwan Ditta, 29, and Mohammed Dilal, 26, of Halifax were later charged with possession of information of use to terrorists, reports AP.
Three men arrested by Greater Manchester Police remain in custody but have not been charged.
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