Fifty six people are before the court on charges of directing the logistical side of the Basque separatist group ETA's battle for an independent homeland.
They allegedly raised funds, forged passports and helped ETA commandos communicate with one another.
The trial is the largest ever in terms of the number of defendants to go before the National Court, the Spanish tribunal which deals with affairs of state and terrorism cases.
The case stems from a probe by Baltasar Garzon, Spain's leading anti-terror investigator.
Those on trial include alleged members of Basque youth groups and organisations and businesses that portrayed themselves as coordinators of pro-independence activities but were banned by Garzon on grounds they were a front for fund-raising and other
support for ETA.
These groups were the "stomach, heart and head" of ETA, prosecutor Jesus Santos said last week.
The testimony section of the trial is expected to last anywhere from four to eight months, according to the national news agency.
The proceedings coincide with a relative lull in attacks by ETA.
The organisation has been blamed for more than 800 deaths since the late 1960s but has not carried out a lethal one since May 2003 – that attack killed two policemen in a car-bombing in the northern town of Sanguesa.
Hopes for peace
The current Socialist government has offered to hold talks with ETA if it renounces violence, raising hopes that an end to the lengthy conflict might be near.
Basque nationalists say the trial can only hinder prospects for peace, and consider it a by-product of the previous conservative government which was in power when the probe began.
Spain, the European Union and the United States classify ETA as a terrorist organisation.