A Spanish judge jailed 17 members of Batasuna – the banned political wing of the armed separatist group ETA.
A group of 23 were arrested in raids in the northern Basque region earlier this week.
They had been detained in the Basque town of Segura while meeting illegally on Thursday, authorities said.
They were jailed following questions by National Court Judge Baltasar Garzon.
The 17, all senior members of Batasuna, were provisionally charged with membership to an armed group, Garzon said in a document.
Eight of them were repeated offenders, a court official said.
Four others were released on bail of €10,000 and the other two men were freed as they were identified as being without direct links to the banned political party.
News reports said most of the detained refused to answer any questions by Garzon.
On Saturday, thousands in the Basque country protested over the arrests. A senior member of Batasuna, Pernando Barrena, said Spain’s government had effectively declared war by arresting most of its leadership.
Prime minister Jose Luis Rodriguez Zapatero pursued a peace process with ETA after the group declared a ceasefire in March last year, and saw Batasuna as a potential bridge to ETA militants.
But the peace talks went nowhere, and Eta shattered its ceasefire with a December bombing that killed two people. The government since has arrested dozens of ETA suspects and is now going after their political allies.
Batasuna’s most prominent leader, Arnaldo Otegi, was jailed on terrorism charges in July.
Barrena called the arrests “kidnappings” and said they were revenge for the movement’s hard-line position in the peace talks.
Batasuna was banned in 2003 in a Supreme Court ruling that barred it from running in elections and holding formal meetings. The party, however, has been allowed to hold news conferences and stage street rallies.
ETA has killed more than 800 people since the late 1960s in a campaign of shootings and bombings. The group is seeking an independent Basque homeland in northern Spain and south-west France.