Three held over 7 July London terror blasts
Last night, they were being questioned at a high-security police station in central London.
Scotland Yard said the three were arrested "on suspicion of the commission, preparation or instigation of acts of terrorism."
Security sources said the arrests related to "unanswered questions" about the attacks in July 2005, which killed 52 people. Among those questions are why the car in which the bombers travelled to London contained spare explosive devices.
An official government account of the attacks on Tube trains and a bus published last year reached no firm conclusions about the purpose of the extra devices. Persistent conspiracy theories suggest the original plot involved more than four bombers.
But one source familiar with the inquiry last night insisted there was "no question" of the arrested men being suspected of direct involvement in the attacks.
Rather, they are believed to have provided support for the four bombers, either by helping finance the operation or by collaborating in the planning or preparation of the attacks.
Following the arrests, officers began searching five addresses in Leeds, some of them in the Beeston area of the city, where two of the attackers lived.
Two addresses in east London were also being searched.
Although the principal members of the 7/7 conspiracy died on the day, police and MI5 officers have continued to investigate the plot.
MI5, the Security Service, has been dogged by questions about exactly what was known of the four 7/7 bombers before their attack.
It emerged in the weeks afterwards that at least two of the bombers had become known to MI5 as a result of a separate counter-terrorism operation.
Some reports have suggested the plotters were regarded as close associates of known terrorist suspects. But Whitehall security sources strongly deny that, insisting the perpetrators' links to the earlier plot became clear only after July 2005.