Three men are being questioned under the Terrorism Act after they were arrested at a Blackwood address.
Gwent Police arrested the three in Blackwood on suspicion of drugs offences on Wednesday.
After further inquiries, the Metropolitan Police Service Counter Terrorism Command was called the next day and transferred the trio to a central London police station.
They were arrested on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.
Two of the men are aged 24 and other is 41.
On Wednesday Gwent Police had originally raided the property in Waunllwyn Crescent, Blackwood, after gaining a warrant under the Misuse of Drugs Act.
On Friday though a bomb disposal unit had arrived on the street.
South Wales Fire and Rescue Service, which was also called to the scene, said several small containers of chemicals were discovered.
Neighbours have been reported as saying that on Wednesday police had asked them to close doors and windows because of found chemicals letting off toxic gas.
A further house on the street has been searched and a van has been removed.