More arrests expected
The detainees, aged between 22 and 34, are suspected of the commission, preparation, or instigation of acts of terrorism.
It is the second wave of arrests within weeks in the long-running investigation into the July 7 attacks.
Three men were charged last month with conspiring with the London bombers.
A security source said: "This is the second phase of arrests ... but there's still plenty to do, and it's not over."
Police said the suspects were being taken to a central London police station to be interviewed by counter-terrorism detectives.
They were also searching five houses in West Yorkshire and two flats in Birmingham in central England.
The security sources said a key focus of investigations into the 2005 attacks is to trace people who may have provided logistical support to the bombers.
Peter Clarke, head of Scotland Yard's counter-terrorism command, said last month he was certain that some people with knowledge of what lay behind the attacks had so far withheld information from police.
He said at the time that more arrests were likely.
MI5, Britain's domestic intelligence service, issued a rare public defence of its operations last week.
The statement came after it emerged that its counter-terrorism agents had taken photographs and recorded conversations of two of the suicide bombers, well over a year before they carried out the attacks.
The agency said the men surfaced as unidentified contacts of a group of men under surveillance in a separate plot, and there was no evidence at the time that the two were involved in terrorist activity in Britain.