Muslim cleric Abu Hamza al-Masri is spending a second day in police custody after being arrested on suspicion of terrorism offences.
Police say they have arrested a 47-year-old man
He was arrested on Thursday morning at Belmarsh prison, in south-east London, where he fights extradition to the US.
Police have taken him to Paddington Green station in central London to question him.
He is being held on suspicion of the commission, preparation or instigation of acts of terrorism.
The offences Mr Abu Hamza is being held over are alleged to have taken place in the UK.
Police can hold him for a maximum of 14 days under the Terrorism Act before they have to decide whether to charge or release him.
A Met Police statement said: "A 47-year-old man was arrested Thursday morning under Section 41 of the Terrorism Act 2000 on suspicion of being involved in the commission, preparation or instigation of acts of terrorism.
"He has been taken into custody to a central London police station where officers from the MPS Anti-terrorist Branch will interview him. He was arrested in the London area."
The Crown Prosecution Service say they are working with the police to see if charges will be brought.
If they are, the charges will be given priority over the US extradition attempt.
The 11 charges the US authorities want Mr Abu Hamza to face include giving advice to those involved in a Yemen kidnapping in which three Britons died in 1989.
Other allegations include attempting to set up a terrorist training camp in the north-western state of Oregon and sending someone to Afghanistan to fight for the Taleban.
But Mr Abu Hamza's legal team say he will not get a fair hearing if he is extradited to the US.
British police say his arrest is not related to the accusations in the US and is "part of separate, domestic inquiries".
He was banned from preaching at Finsbury Park, north London, last year but he continued to address his supporters from the pavement.