BILBAO, SPAIN – Hundreds of people protested in Bilbao, northern Spain, on Tuesday against the arrest a day earlier of 18 leaders of an association that aids jailed members of the armed Basque separatist group ETA.

The demonstrators rallied in a square in the center of the Basque city to protest against the police operation against Herrira, which was founded in February 2012 to help ETA prisoners and their families, and demanded that the 18 people arrested be released.

The protest was organized by pro-independence Basque groups, who called for another demonstration Saturday against the police operation in Bilbao, the Basque region’s economic capital.

Police detained the Herrira leaders Monday in simultaneous raids carried out in Bilbao, Hernani, Pamplona and Vitoria. They also closed down 38 websites run by the group as well as dozens of Facebook and Twitter accounts, and froze its bank accounts.

The Spanish government accuses Herrira of “organizing and supporting demonstrations that praised ETA prisoners and their ideals” and it argues that the group had replaced two other associations that aid ETA prisoners that had been banned in Spain.