Scotland Yard has confimed that two Spanish nationals are being questioned in connection with alleged terrorist offences in Spain.
The men, aged 39 and 55, were arrested by the counter terrorism unit about 4am on Friday.
Both men, who are belived to be members of the Basque terrorist group, were held under the Extradition Act after being arrested at a residential address in west London.
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.
Officials say the arrests are not linked to the Olympics, which are less than a month away.