Radical cleric Abu Qatada has been arrested for allegedly breaching his bail conditions, days ahead of a new government bid to have him deported.
He was arrested by UK Border Agency officials on Friday, a day after raids in London by counter-terrorism police.
Officers searched two homes and a search of a third property was ongoing.
On Monday, the government will go to the Court of Appeal in a bid to overturn a judge's decision to allow Abu Qatada to stay in the UK.
A Home Office spokesman said: "The UK Border Agency arrested a 52-year-old man from north London for alleged breaches of his bail conditions imposed by the Special Immigration Appeals Commission (Siac)."
He added that the case would be considered by Siac at the earliest opportunity.
No arrests have been made in connection with the counter-terrorism operation.
A Metropolitan Police spokesman said: "Searches at a residential address in north-west London, a residential address in west London and a business address in north-west London have been completed.
"A search at a residential address in north-west London is ongoing."
Abu Qatada has been found guilty of terrorism offences in his absence in Jordan, where he faces a retrial for allegedly conspiring to cause explosions targeting Western and Israeli targets before the year 2000.
But Siac ruled that Jordan had not proved Abu Qatada's retrial would be free of evidence obtained by torture.
Home Secretary Theresa May will challenge this decision in front of three Court of Appeal judges led by Lord Dyson, the Master of the Rolls, on Monday.
Abu Qatada was granted bail following the decision by the Siac judges in November last year and released from Long Lartin prison in Worcestershire.