A seventh person has been arrested in connection with the Parsons Green Tube bomb attack on 15 September.
The 20-year-old man was arrested in Cardiff at 06:00 BST under section 41 of the Terrorism Act and is now being held at a south London police station.
Last week Ahmed Hassan, 18, of Sunbury, Surrey, was remanded in custody charged with attempted murder.
A bomb partially exploded on a train at the Tube station in south-west London during rush hour, injuring 30 people.
Three other men arrested over the attack have been released without charge; two further men, aged 25 and 30, remain in custody.
Monday morning’s arrest was carried out by officers from the Metropolitan Police’s Counter Terrorism Command, supported by the Welsh Extremism and Counter Terrorism Unit (WECTU) and South Wales Police.
Police are searching an address in Cardiff and say another search is ongoing at an address in Surrey.
Mr Hassan, who was arrested in Dover, Kent, on 16 September, is due to appear at the Old Bailey on Friday 13 October.
He is also accused of causing an explosion likely to endanger life or cause serious injury.
On Friday, Westminster Magistrates Court heard the bomb, which failed to detonate properly, had been packed with knives and screws.
Prosecutors said Mr Hassan had intended to kill innocent people because of his “warped political view”.