A counter-demonstration of several thousand protesters was held in Madrid to oppose negotiations with militant Basque separatists. The new party, whose name will be revealed Tuesday, hopes to compete in May regional elections as a successor to Batasuna, the political wing of armed separatist group ETA that was banned in 2003.
But first the central government in Madrid must approve a petition that the new party be included on electoral lists. The party plans to present the petition to the Interior Ministry on Wednesday.
The establishment of the new party follows the declaration of a permanent cease-fire by ETA on Jan. 10.
The rallies Saturday were held throughout the Basque region, including in San Sebastian and Bilbao.
"The actions are a means of making visible in Basque towns and cities the support for a necessary legalization process," Niko Moreno, mayor of the Basque town of Elorrio, told The Associated Press. He said that around 15 percent of the Basque population would vote for a successor to Batasuna if it were to be legalized in time for the elections.
The Spanish government has insisted that an ETA cease-fire is not enough to legalize a separatist party containing militants with direct links to that violent organization.
ETA is considered a terrorist organization by Spain, the European Union and the U.S. It has killed more than 825 people since the late 1960s, but has recently been decimated by the continued arrests of its leaders and by dwindling grass roots support.
Interior Minister Alfredo Perez Rubalcaba has repeatedly said ETA must disband and disarm, and the politicians of Batasuna must forever renounce the use of and support for terrorist violence.
Txelui Moreno, who used to be a spokesman for the former Batasuna, said the new party will comply with the Spanish government's requirements.
"We are going to respect the laws that apply in the Spanish state in order to be legal," he said. "This is the good path to achieve peace in our country."
The organization Voices Against Terrorism, led by former Interior Minister Jaime Mayor Oreja, marched in Madrid late Saturday to protest against allowing anyone linked to ETA access to public office unless they renounce violence publicly.
Several thousand protesters carried banners saying, "No more lies, no more false cease-fires" and chanted "We are always on the side of the victims of terrorism."
ETA has declared 11 cease-fires before this most recent one in January. The previous "permanent cease-fire" was called in 2006 but it ended months later with a huge car bombing claimed by ETA that killed two people at Madrid's international airport.
Spain's ruling Socialist Party had begun tentative negotiations with ETA before that deadly explosion which many observers said was an expression of frustration by Basque militants at a lack of progress in talks with the government.
In 1998, when Mayor Oreja was interior minister in the Popular Party government - now in opposition - he too was forced to suspend negotiations with ETA when it ended a cease-fire it had called to try and obtain concessions from the government.
Associated Press writer Harold Heckle in Madrid contributed to this report.