The widow of one July 7 bomber, Hasina Patel, and her brother, Arshad Patel, are amongst people arrested in unarmed raids. They are currently held in London on suspicion of commissioning, preparing or instigating acts of terrorism.
Ms Patel's representative, Suresh Grover, said he is "shocked and outraged" at the arrests. Seven addresses in the West Midlands and West Yorkshire are also being searched.
The police are currently trying to ascertain who knew and who helped the bombers that killed several people and left more than 750 injured almost 2 years ago in attacks on London.