London (CNN) -- Two members of the Basque separatist movement ETA, including one who has been convicted of more than 20 murders, were arrested Friday in London over alleged terror offenses, the Spanish Interior Ministry said.
The men were detained at a west London home by officers from the Metropolitan Police's Counter Terrorism Command.
The Spanish Interior Ministry named them as Antonio Troitino Arranz, 55, and Ignatio Lerin Sanchez, 39.
Troitino has been part of the Basque separatist movement ETA since the 1980s and was involved in many terrorist attacks, the ministry said.
One of the worst was the bombing of a police bus in Madrid that left 12 officers dead and injured 51 people, including civilians, it said. Troitino was the one who triggered the bomb, the ministry said.
Troitino is wanted under a European arrest warrant issued in April last year "for belonging to an armed group or terrorist organization," it said.
According to Spanish media reports, Troitino was let out of jail due to what is described as a judicial error, and fled before it could be corrected and he could be detained again.
Ignacio Lerin, who has been on the run since 2007, when his brother was arrested as an ETA member, is wanted on suspicion of belonging to an armed group and possession of explosives, it said.
The arrests have dealt a blow to ETA, the ministry said, as Troitino has direct links to the group's present leadership.
In London, officers continue to search the house in Hounslow where the two were arrested before dawn, a Metropolitan Police statement said. A search of a second home has been completed. No weapons have been found, the statement said.
The two suspected ETA members, whose arrests were made under extradition law, are being held at a central London police station.
A 38-year-old woman also was arrested at the same house as the two Spaniards. She is accused of fraud and remains in police custody, the police statement said.
Two other men at the same address were arrested for alleged immigration offenses.
Spanish security forces have arrested 16 members of ETA since January, including three in the past week, the Interior Ministry said.
Listed as a terrorist organization by Spain, the United States and the European Union, ETA is blamed for more than 800 deaths in its decades-long fight for an independent Basque state that it wants carved out of sections of northern Spain and southwestern France.
Last year, ETA announced "a definitive cessation of its armed activity," raising hopes that decades of separatist violence may finally be over.
Spain's conservative government has stepped up arrests since winning elections in November. The Popular Party of Prime Minister Mariano Rajoy has long maintained a tough line against ETA.
CNN's Alexander Felton and Al Goodman contributed to this report.