Four men have been arrested on Majorca and two more in the UK and Germany as part of a Spanish investigation into support for so-called Islamic State, police say.
The suspects are said to have produced and spread violent videos to recruit would-be jihadist fighters online.
One was a 43-year-old Islamic preacher from Birmingham.
He was arrested in the city's Sparkhill area under a European Arrest Warrant and later appeared in court in London.
The man was identified as Tarik Chadlioui, a Belgian-Moroccan father of eight children who arrived in the UK in 2015. As he challenged a Spanish extradition request, the judge told him he was alleged to be a member of a terrorist organisation who was actively engaged in terrorist activities.
He was remanded in custody until 5 July.
All six in detention were described as having Moroccan or dual Moroccan nationality, Efe news agency reported. The four held on Majorca were flown by helicopter to Madrid where they were due to appear in court.
Police in western Germany said they had detained a 28-year-old Spanish national in Dortmund, emphasising there was no indication of any planned attack in Germany itself. A judge will decide in the coming weeks whether to extradite him.
The Spanish investigation dates back to 2015 when a series of videos were posted online showing how a young Muslim in Spain was indoctrinated, recruited and eventually sent to fight in Syria.
Spanish authorities said the Salafist preacher based in Birmingham was known to European police and intelligence and they alleged he had travelled to Palma de Mallorca to steer the jihadist cell.
The group was based in Majorca and organised secret weekly meetings aimed at recruiting young people to travel to conflict zones to fight, the Spanish interior ministry said.
It added that Spain had arrested 178 "jihadist terrorists" since June 2015.
Last week, Spanish authorities detained three suspects in Madrid, one of whom was said to be an "extremely dangerous" man who was planning an attack in the city.
Spain suffered one of the deadliest jihadist attacks on European soil when bombers murdered 192 people on four crowded commuter trains at Atocha station in Madrid.