Arnaldo Otegi, the leader of the banned Basque party, Batasuna, is facing another jail term after taking part in a separatist protest in San Sebastian.
Otegi is in court on Monday accused of being an apologist for the Basque terrorist group, ETA.
On Friday he was freed from prison on bail of a quarter of a million euros. He was detained last month on charges of inciting violence during a recent strike in the Basque country.
But a judge decided to release him to allow him to take part in any peace talks with Basque militants who began a permanent ceasefire two weeks ago.
Batasuna party is considered the political wing of ETA, which waged a violent 38-year campaign for independence for the Basque region, resulting in the death of some 800 people.