Searches are being carried out at 12 addresses
A ninth person has been arrested in Birmingham over what senior security sources say was an alleged plot to kidnap a member of the armed forces.
Police made the arrest this afternoon on a motorway in the city area. Eight men were arrested earlier following a series of morning raids.
Police said their investigation may take "days, if not weeks".
Security sources said a terror plot not aimed at mass casualties would mark "a different approach" to UK terrorism.
A total of 12 addresses in the Sparkhill, Washwood Heath, Kingstanding and Edgbaston areas have been sealed off and are being searched following raids at about 0400 GMT, say police.
Eight men were arrested at separate addresses and are being held at a West Midlands police station.
In a press conference at about 1530 GMT, Assistant Chief Constable David Shaw, of West Midlands Police, said the ninth arrest had been made just minutes earlier.
"I think that illustrates to you that this remains a dynamic, fluid operation and this is by no means finished," he said.
The search of all the premises would take "some days" to complete, he added.
He said police were at "the foothills of what is a very, very major investigation for us" and officers were actively seeking help from the community, in particular from Muslims.
Community advice lines in a range of languages were being set up and 5,000 leaflets will be distributed to local people, he added.
"We think is it is critical that we involve them in what is going on as much as we can," he said.
Muslim community leader Shabir Hussein said the community should co-operate with the police and remain vigilant of unusual activity in their homes.
"The community is under stress," he said.
"They do not know where their children are, they do not know what they do. At the mosque we are saying 'open your eyes, look underneath your feet'."
Earlier BBC Home Affairs Correspondent Daniel Sandford said sources close to the investigation said the aim of the alleged plot was to kidnap a serving member of the armed forces, perhaps while they were on home leave.
He stressed the arrests were based on intelligence, which could prove to be wrong.
The Ministry of Defence said it could not confirm or deny reports of such a plot.
Two houses and a general store in Alum Rock, near Washwood Heath, were raided.
The Maktabah book store in Stratford Road, Sparkhill, which is believed to sell Islamic literature, was also raided.
'Anger and cynicism'
Police have cordoned off roads around Jackson Road and Foxton Road, in Alum Rock, and Poplar Road and Stratford Road, in Sparkhill.
The BBC's Phil Mackie said there had been some anger and cynicism among locals who had witnessed previous terror raids in the area without anyone being charged.
Saqib Hussain, of Sparkhill, said he had visited the raided book shop many times and "had never seen any suspicions of terrorist activity".
Mohamed Barber told BBC News his cousin was one of the men arrested in Alum Rock.
"We can vouch for him he is innocent. He doesn't even have time to go to Friday prayers - that's how busy he is," he added.
Abdul Ghaffoor, a regular shopper at the general store, said he had never heard politics being discussed in the shop.
Leaders at the Alum Rock Islamic Centre, the main mosque in the area, said the community was shocked at events, but urged calm and appealed to people to co-operate with the police investigation.
Ayub Pervaz, the mosque's president, said: "If people have broken the law they should be brought to justice.
"But we also appeal for no trial by media. If any of those arrested turn out to be innocent, this should be made clear."
The eight arrested following the morning raids are suspected of "the commission, preparation or instigation of acts of terrorism".
Home Secretary John Reid urged restraint and described it as a "major operation".
The Midlands Counter Terrorism Unit ran the operation with the West Midlands Police, the Metropolitan Police and MI5.