Those arrested on Monday include 12 lawyers and four former treasurers of Basque prisoner support groups, including Amaia Izco, the national spokesperson for the leftist Basque party, Sortu.
They are accused of fraud against Spanish tax authorities and money laundering.
According to Spanish media, the operation was carried out in the Basque Country, Pamplona and Madrid on Monday morning.
Most of the lawyers arrested were in Madrid; their detention has casued a halt to the trial of 35 members of the banned Basque nationalist party Batasuna who were due to appear before the National Court because of their alleged links to ETA.
ETA, an armed separatist group that seeks an independent Basque state in north-eastern Spain and southern France, has launched attacks against the Spanish state since 1961.
In 2011 ETA announced an end to its "armed activities."