The wife of 7 July bomber Mohammad Sidique Khan is among four people being questioned in connection with the 2005 attacks, which killed 52 people.
Mohammad Sidique Khan bombed a Tube train at Edgware Road
Officers arrested Hasina Patel, 29, her brother Arshad Patel, 30, and Khalid Khaliq, 34, in West Yorkshire; Imran Motala, 22, was held in Birmingham.
They are being held in London on suspicion of commissioning, preparing or instigating acts of terrorism.
Seven addresses in the West Midlands and West Yorkshire are being searched.
Suresh Grover, who has represented Ms Patel since 2005, said he was "shocked and outraged" at the arrests.
"Hasina and her family have co-operated with the police, intelligence services, with the coroner, they have been in regular contact with everyone," he said.
"There is... absolutely no reason for these dramatic arrests to take place in this fashion."
BBC home affairs correspondent Daniel Sandford said Ms Patel's representatives insisted she had simply been trying to rebuild her life.
Forensic teams are searching a property in the Handsworth area of Birmingham, in Leonard Road, and police are guarding a block of student flats in Selly Oak.
But a University of Birmingham spokeswoman said Mr Motala, who was arrested in the Selly Oak area, was not a student there.
A police lorry removed a silver Peugeot 307 from the flats - in a block called Victoria Hall on Grange Road. A car was also removed from the Handsworth address.
BBC sources say Mr Motala is also related to Ms Patel.
In West Yorkshire five addresses are being searched - two houses in Pentland Road and Dale Street, Dewsbury; two in Tempest Road, Beeston, Leeds; and one in Hayburn Gardens, Batley.
Mr Khaliq is from Tempest Road in Beeston - the street where 7 July Aldgate Tube bomber Shehzad Tanweer lived.
Tanweer was one of four suicide bombers to mount the co-ordinated attacks in the capital nearly two years ago, which also injured more than 750 people.
Bomber Shehzad Tanweer also lived in Tempest Road in Beeston
Ringleader Khan, 30, killed six people when he bombed a train near Edgware Road station.
A train travelling between King's Cross and Russell Square was also bombed, as was a double-decker bus in Tavistock Square.
The arrests on Wednesday morning followed an intelligence-led operation involving Scotland Yard's counter-terrorism command and units in West Yorkshire and the West Midlands.
Armed officers were not used in the operation.
Police said the inquiry remained a "painstaking investigation" into whether anyone knew what the bombers had been planning and who may have helped them.
Both West Midlands and West Yorkshire forces said they were keeping the local communities informed and that meetings had taken place with councillors and other key figures in the area.
Chief superintendent Barry South, of West Yorkshire Police, said it was a "high profile investigation, but a low key response" in terms of policing.
"We don't perceive any threat from the addresses that we are searching at this moment," he added.
Mr South would not confirm the names of the suspects who are being held at London's Paddington Green police station.
Last month the first people to be charged in connection with the bombings appeared at the Old Bailey in central London.
Mohammed Shakil, 30, Sadeer Saleem, 26, and Waheed Ali, 23, of Beeston, Leeds, are accused of conspiring with the four bombers to cause explosions.
The trio, who came before the court via a video link from prison, were remanded in custody until 8 June.