Police are continuing to question 14 people arrested under the Terrorism Act in raids on Tuesday.
Police officers stormed a number of addresses at dawn on Tuesday
The arrests were made in raids on homes and businesses in London, Cambridge, Walsall, Dudley and Luton.
Six arrests were in Cambridge and east London, four in south-west London and four in the West Midlands and Luton.
Police have been given extra time - until Friday - to question the four from south-west London and three arrested in Cambridge and east London.
The Cambridge and London arrests were made under section 41 of the 2000 Terrorism Act, which refers to commission, preparation or instigation of acts of terrorism.
The four held by West Midlands Police were arrested under the same Act, but no details were given of which section.
The four arrested in south-west London were not named, but their ages were given as 28, two of 29 and one of 32.
The six arrested in Cambridge and east London were described as two men aged 27, one of 31, one of 34, and two women, aged 18 and 20.
Police have an extension until Friday to question the two 27-year-olds and the 34-year-old.
No such extension has yet been granted for the 31-year-old man and the two women, but they remain in custody at present.
Of the four arrested by West Midlands police, two - aged 23 and 24 - were arrested in Dudley; one, aged 26, in Walsall, and another 26-year-old in Luton.
Britain's most senior police officer, Sir John Stevens, said the arrests were part of an "ongoing operation".
"We are arresting people continuously," the Metropolitan Police Commissioner added.
"It is part of this massive effort we have been having since 11 September. And it will continue."
The latest operations come as Prime Minister Tony Blair warned that the UK must remain "permanently vigilant" against the threat of terrorism.
Britain has been on the second highest security alert for two weeks since officials warned they had intelligence suggesting an attack was planned.
They have not specified any particular target.