Six men arrested in connection with one of the most significant intelligence-led counter-terrorism operations this year remain in police custody.
The men were detained at or near their homes in Birmingham on suspicion of the commission, preparation or instigation of an act of terrorism in the UK following a joint investigation by police and MI5.
It is understood the investigation relates to suspected Islamist extremism, but it is not thought that an attack or threat was imminent.
A seventh person, a 22-year-old woman, was arrested on suspicion of failing to disclose information contrary to the Terrorism Act 2000, police said.
West Midlands Police said the "large-scale operation" had been running for some time and had been subject to regular review, adding that the action was necessary "in order to ensure public safety".
All the arrests, which are are thought to be among the most significant from anti-terror operations this year, were made by unarmed officers.
The six men, aged between 25 and 32, were taken into custody in the Moseley, Sparkbrook, Sparkhill, Ward End and Balsall Heath areas of Birmingham between 11.30pm on Sunday night and 1am on Monday.
The 22-year-old woman, from Saltley, Birmingham, was arrested at 6.30am on Monday on suspicion of failing to disclose information contrary to the Terrorism Act 2000. With the exception of the woman arrested, all are under terrorism detention.
Assistant Chief Constable Marcus Beale said: "I fully understand that after a counter-terrorism operation of this size, the public will want to know more details about what we suspect may have been going on - and what has prompted us to take this action.
"For us, once the key issue of public safety has been addressed - and it has been for this operation - the main thing is to do all we can to investigate the matter professionally, thoroughly and fairly."