Controlled explosions took place at a Yorkshire farm
Officers obtained permission from magistrates at 1700GMT on Monday to hold the men for a further 48 hours.
The suspects are being questioned about bombings in west London, including an attack on the BBC, and an explosion in the centre of Birmingham on 3 November.
One of the men, a 26-year-old, was arrested at Woodall services on the M1 in South Yorkshire on Saturday afternoon and is being questioned in Leeds.
The other six were arrested in London and Liverpool last Thursday, and are being questioned at a central London police station.
They are all aged between 20 and 41 and one is thought to be from the Irish Republic, while the rest are believed to be from England.
Since those arrests, officers investigating the bomb attacks have arrested an eighth person.
The 19-year-old was arrested in north Lincolnshire on Sunday and taken to Leeds for questioning under the Terrorism Act.
Officers are still examining the disused pig farm at West Ardsley, Yorkshire, where a series of controlled explosions were carried out on Thursday.
In a separate inquiry, police have seized weapons and arrested a number of people in County Armagh.
The items were found during a search in the nationalist Kilwilkie estate in Lurgan.
They include explosives, a detonator, a rifle, two handguns, ammunition and masks.
Controlled explosions
The West Ardsley farm has been searched as part of the investigation into the 18-month bombing campaign.
Residents of 20 nearby houses were allowed home on Friday following the investigation by Army and bomb disposal experts.
In a statement, Assistant Chief Constable Grahame Maxwell said the operation had been difficult but that their priority was to ensure the community's safety.
Addresses in London, where five of the suspects were arrested, and in Liverpool were also searched by forensic specialists but detectives would not say if anything had been found.
Under the Terrorism Act, police have 48 hours to question suspects but can apply for extensions up to a total of seven days.
Appeal
Police are keen to hear from anyone who was in the area at the time, especially in a cafe next to the garage, the Park Brook pub or a nearby church on Wolverhampton Road.
Detectives have renewed their appeals for information about the attack, and released more details about the car used.
The E-reg gold or beige Audi Coupe was bought earlier that day from Junction 10 Car Sales, on Wolverhampton Road, Walsall, before being left in Smallbrook Queensway.
Cash was paid for the car and it left the premises, just off the M6, between 1415 GMT and 1430 GMT.