Two men suspected of being senior members of the Basque separatist group Eta have appeared in court after being arrested in west London by armed police.
Antonio Troitino Arranz and Ignacio Leron Sanchez were held in connection with alleged terrorist offences in Spain.
Both men were detained under the Extradition Act after being arrested at a residential address in Hounslow in the early hours of Friday, police said.
They appeared at Westminster Magistrates Court where they were remanded in custody until July 20.
The arrests were made by officers from Scotland Yard's counter terrorism command, supported by armed police and officers from the force's extradition unit.
The court heard that Sanchez is suspected of being involved in terrorist attacks after the Spanish authorities raided his home in March 2007 and found 150kg of explosives, bomb-making equipment and Eta literature.
The 39-year-old, who had fled before the raid, faces a maximum sentence of 12 years if is he is convicted.
Prosecutor Katherine Tyler told the court Arranz, 55, was part of a terrorist cell which planned a car bomb attack on a civil guard patrol on July 14, 1986.
He was jailed in 1989 for the attack, which killed 12 officers, and was released in April last year after serving 24 years of his sentence.
The London hearing was told that a court later revoked his release and a warrant was issued for his arrest, by which time he had already gone on the run.
A 38-year-old woman was arrested at the same address as the two men for alleged fraud offences. 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.