Tens of thousands of people in the northern Spanish city of Bilbao have defied a government order and protested against the ban for Basque separatists to stand in local elections.
The protest march was conducted in silence
The peaceful march was called by Udalbitza, a self-proclaimed assembly of elected officials which promotes Basque independence.
Udalbitza is considered a successor to the Batasuna party, which has been permanently banned because of its alleged links with the armed Basque separatist group, ETA.
In a separate development, French police detained four suspected members of ETA - a move that was hailed by Spain as a major blow against armed Basque separatists.
The rally was conducted in silence, with demonstrators carrying banners that read "Yes to Udalbitza, yes to the Basque country."
Batasuna could now be subject to sanctions in the US
They protested against the detention of several Batasuna members on the orders of Spain's anti-terror Judge Baltasar Garzon.
The demonstration was also against Friday's decision by Spain's Constitutional Court confirming the cancellation of electoral lists for over 1,000 Basque political candidates in the 25 May poll.
Batasuna was banned in Spain on the grounds that it is part of the "terrorist network" of ETA - a charge Batasuna members vehemently deny.
Earlier this month, the United States added Batasuna to its list of terrorist groups.
The decision, which makes the group liable to sanctions in the US, came after pressure from Spanish Prime Minister Jose Maria Aznar.
Meanwhile, the police in France said the four detained ETA suspects - three men and a woman - were allegedly carrying weapons, when they were stopped late on Friday in the south-western town of Saintes.
The Spanish Interior Minister, Angel Acebes, hailed the arrests as a major success.
He said one of the detained was regarded as a leader of ETA's commando units and accused of having taking part in three murders, including that of a Basque police officer in 2001.
ETA - whose full name Euskadi Ta Azkatasuna stands for Basque Homeland and Freedom - first emerged in the 1960s as a student resistance movement bitterly opposed to General Franco's repressive military dictatorship.
In subsequent decades the armed organisation has waged a bloody campaign for independence for the seven regions in northern Spain and south-western France that Basque separatists claim as their own.