MADRID (AFP) -- Spanish authorities said that they smashed a commando unit of the armed Basque separatist group ETA and seized large quantities of bomb-making materials.

The announcement came just hours ahead of a meeting by ETA's banned political arm, Batasuna, which was attended by 15,000 people in the northern Basque city of Bilbao.

The commando unit, known as "Donosti" and based near San Sebastian, is suspected of being "behind 24 attacks in Spain between 2004 and 2006," Spanish Interior Minister Alfredo Perez Rubalcaba said.

Materials to make around 170 kilos (375 pounds) of explosives were recovered and eight people arrested including the suspected head of the unit, Jose Angel Lerin Sanchez.

The police operation began on Wednesday and was one of the biggest since ETA's "permanent ceasefire" declaration in March 2006.

Computer equipment was also seized and files were being analysed. More arrests and seizures could follow, the interior ministry said in a statement.

In one of the raids carried out on Friday night in the Basque country and Navarra, around 140 kilos of explosive materials were recovered in an apartment in the town of Berriozar.

The Basque prisoners support group Askatasuna said the apartment belonged to Lerin, 35, who had been on the run since 2005.

Lerin was one of those arrested earlier this week in possession of handguns stolen in an October robbery on a weapons factory in Vauvert, France.

The interior ministry does not believe the unit was behind ETA's December 30 blast at Madrid airport which killed two people.

That attack -- the first time ETA had killed anyone since May 2003 -- was carried out by a French-based group, the ministry said at the time.

The blast dramatically ended ETA's nine-month-old unilaterally declared ceasefire and led Spanish Prime Minister Jose Luis Rodriguez Zapatero to break off all contact with the militants.

News of the raids came as Batasuna's meeting in Bilbao went ahead as planned, despite a Spanish court ruling a day earlier banning the gathering.

The court had outlawed the meeting because it was scheduled to be used to unveil the political platform of a new party created by Batasuna to contest local elections on May 27.

But authorities said on Saturday said it could go ahead as long as organizers pledged in writing that the new party called Abertzale Sozialisten Batasuna (ASB - Socialist Patriotic Unity) would not be mentioned.

The organizers duly made this pledge, so allowing the gathering to go ahead, Spanish radio said.

Batasuna leader Arnaldo Otegi told the Bilbao meeting the separatist left would be represented in the May poll.

"The pro-independence left has this right and will be present (at the election) without disguise," he told the crowd at the Bilbao Exhibition Center.

Otegi did not say how Batasuna would take part in the poll as the new ASB party will probably also be banned within the next weeks.

"What is at stake in this election is the issue of a definitive settlement of the conflict between the Basque country and the Spanish state," he added. Those who deny Batasuna a role in the election "reject the democratic process" to end the Basque conflict, he said.

"We realize that a lot of people are panicking at the idea of a truly democratic solution to this conflict."

A spokesman for Batasuna -- itself banned since 2003 from taking part for failing to formally break with ETA's armed campaign – had said the meeting was not to "present the electoral platform" of the new party but was to make a "democratic proposition to resolve the Basque conflict."

ETA is blamed for 819 deaths in its 38-year campaign to win independence for the Basque region. Both the European Union and the U.S. State Department consider it a terrorist organization.