Two Spanish nationals, believed to be members of Basque separatist group Eta, have been arrested in connection with alleged terrorist offences, police said.
The men, aged 39 and 55, were held by officers from Scotland Yard's counter terrorism command in connection with alleged terrorist offences in Spain.
Both men were held under the Extradition Act after being arrested at a residential address in west London, police said.
Armed officers joined those from the counter terrorism command when the arrests were made at about 4am today, police said.
Searches at two addresses in west London are under way and the men are being held at a central London police station.
The arrests are not linked to the Olympics, which are less than a month away, police said.
A 38-year-old woman was also arrested for alleged fraud offences and two other men were held over alleged immigration offences.
The arrests come after a suspected Basque terrorist lost his application to appeal to the Supreme Court against extradition to Spain where he is accused of the attempted assassination of the Spanish king.
Lawyers for Eneko Gogeaskoetxea Arronategui, 44, argued that his case raised questions of law of public importance that should be considered by the highest court in the land.
But the High Court in London rejected the submission on Wednesday and refused permission for a last-ditch appeal to the Supreme Court.
Last month the High Court overturned a magistrates' court decision that had blocked Arronategui being extradited on the specific charge of attempting to kill King Juan Carlos in 1997.