Garikoitz Ibarlucea Murua is accused of several petrol bomb attacks
Garikoitz Ibarlucea Murua is accused of a series of petrol bomb attacks between 1999 and 2002 in the turbulent Basque region of northern Spain.
He is alleged to have taken part in the bombings of the homes of two town councillors, the offices of a daily newspaper and social security department, and a railway station.
It is the third such arrest in a week - a suspected Eta member was stopped at Lisbon airport on Friday, while another was caught in Normandy last Sunday.
Ibarlucea, 29, was detained in Soho, London, and is set to appear at the City of Westminister Magistrates' Court on Monday.
He will face deportation proceedings.
Scotland Yard said: "Garikoitz Ibarlucea Murua was detained on behalf of the Spanish authorities under a European Arrest Warrant alleging 'terrorist offences'."
Ibarlucea was first arrested by Spain's paramilitary Civil Guard in October 2002.
After being held in prison for four months, he was released on provisional bail by an investigating judge at the National Criminal Court.
But he failed to turn up for his trial in October 2006 and an international warrant for his arrest was issued two months later.
Eta has waged a 40-year campaign for independence in the Basque Country, an area that stretches across the French-Spanish border.
The group has been blamed for 828 deaths since its separatist campaign began.
This year, more than 30 suspected Eta members have been arrested, many in France which has long been used as an Eta hideout.