Six men have been arrested in connection with one of the most significant intelligence-led counter-terrorism operations this year.
The men were detained at or near their homes in Birmingham overnight on suspicion of the commission, preparation or instigation of an act of terrorism in the UK following a joint investigation by both 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".
Assistant Chief Constable Marcus Beale said: "As a result of one of those reviews yesterday we decided that the time was right and moved to make six arrests during the night, with a further one a few hours later."
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 last night and 1am today.
The 22-year-old woman, from Saltley, Birmingham, was arrested at 6.30am 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.
Mr Beale went on: "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."
Speculation over potential targets or the identity of the suspects was "neither fair to them or to any judicial process which may or may not follow", he added.
The suspects, who were all from the Birmingham area and arrested at home addresses or nearby, are being held for questioning at an unnamed police station in the West Midlands Police area.
Specialist police teams are searching a total of 14 properties, including the suspects' homes, and computer equipment has already been removed for examination.
Mr Beale said: "We worked hard from early this morning to inform communities in the vicinity of the relevant addresses, stakeholders and partners about the action police officers were taking - and the feedback we have received so far has been supportive."
Police were believed to be searching a house in the Sparkhill area of Birmingham as plain-clothes police officers and forensic teams could be seen at a house in Turner Road.
Meanwhile, a grey estate car was covered with tarpaulin and driven away on the back of a flatbed truck in nearby Ladypool Road.
Officers were also seen going in and out of a large house in nearby Moorcroft Road in Moseley.
West Midlands Police have refused to confirm the addresses of properties which are being searched in the city as part of the investigation.
But the arrests were not related to the Liberal Democrats' conference, which is taking place at the city's International Convention Centre, police said.
The international terror threat level to the UK was reduced from severe to substantial in July, where it remains, but a terrorist attack is still a strong possibility and might well occur without further warning.
There are five levels of threat, ranging from low, meaning an attack is unlikely, to critical, when an attack is expected imminently.
The threat level was last at critical in June 2007, following the attack on Glasgow Airport and the failed car bombings in central London.
:: Anyone with information to assist the inquiry, which is being led by the West Midlands Counter Terrorism Unit, is asked to call the confidential Anti-Terrorist Hotline on 0800 789 321.