Four men are being questioned by UK police investigating a Sunday newspaper's claims that a group of businessmen were trying to buy radioactive material.
The men are being held at Paddington Green station
Three men were held on Friday after Met Police anti-terrorist branch officers targeted a hotel in Brent Cross, London.
The fourth was arrested later at his north London home. The arrests followed a tip-off from the News of the World.
A BBC correspondent said police had not found any radioactive or bomb-making material but that searches were ongoing.
The men have been arrested on suspicion of commissioning, preparing or instigating acts of terrorism.
BBC Home Affairs correspondent Margaret Gilmore said: "The allegation is that these men, who we understand are all businessmen, were trying to buy radioactive materials.
"The suspicion, and this is what the police are questioning them about now, is that they were trying to get hold of radioactive materials to sell them on to terrorists."
'Not highly significant'
She said although searches of houses and premises were still going on, police had not found any radioactive material or a "bomb in the making".
"They don't believe they have arrested highly significant people, on the other hand they have possibly stopped something in the making," she added.
The newspaper claimed it sent in an undercover reporter posing as a Muslim extremist following a tip-off that a Saudi sympathetic to "the Muslim cause" was willing
to pay £300,000 for a kilo of powerful, radioactive "red mercury".
The chemical is said to have been developed by Russian scientists for "briefcase nuclear bombs", although scientists are divided over whether any
According to the News of the World, meetings were held with gang members
hoping to supply the radioactive material to their Middle Eastern buyer.
Gary Thompson, associate editor of the News Of The World said the story followed covert investigations by its reporter Mazher Mahmood.
"He alerted police who made the arrests. We do not know what specific details there were of any targets (for bombs)," he said.
Police marksmen and surveillance teams surrounded the Holiday Inn hotel in Brent Cross on Friday and the
arrests were made.
Police have 14 days to question the suspects, who are being questioned at Paddington Green police station in central London, after which they must be charged or released.
A Metropolitan Police spokesman said on Saturday: "Several addresses have been searched. Some searches continue."
MI5 were also involved in the operation, it has been revealed.