British police arrested six men on Sunday night as part of a large, intelligence-led anti-terrorism operation.
The men, aged between 25 and 32, were taken into custody in Birmingham, and are being held on suspicion of the commission, preparation or instigation of an act of terrorism.
The BBC reported that the arrests are believed to be the most significant to date this year.
It is understood the investigation, which has involved MI5, relates to suspected Islamist extremism.
Unarmed officers are understood to have detained the men in the Moseley, Sparkbrook, Sparkhill, Ward End and Balsall Heath areas of Birmingham.
A woman aged 22 was also arrested “on suspicion of failing to disclose information contrary to the Terrorism Act 2000”.
The Guardian reported that the suspects were being questioned at an unnamed police station in the West Midlands area.
Specialist police teams were reportedly searching their homes, along with several other properties in Birmingham.
West Midlands Police Assistant Chief Constable for security Marcus Beale said:
The operation is in its early stages so we are unable to go into detail at this time about the nature of the suspected offences.
However, I believe it was necessary to take action at this time in order to ensure public safety.
The nature of the suspected offences has not been disclosed.