( AFP ) - A top member of banned Basque separatist party Batasuna said Saturday the arrest of most of the party's leadership was "a declaration of war" by the Madrid government.
The strong reaction was reflected in demonstrations later Saturday as thousands of people took to the streets in Basque cities to protest the government's action.
Pernando Barrena, the most senior official of the political wing of the armed ETA group still at liberty, said the government of Prime Minister Jose Luis Rodriguez Zapatero wanted to "slam the door" on the Basque independence movement.
"The repression will not make us back down" from the fight for independence, he said.
Barrena, accompanied by some 80 Batasuna militants, was speaking at a press conference at a hotel in the port city of San Sebastian in northern Spain.
Across the Spanish Basque Country late Saturday, several thousand protestors demonstrated against the arrests. Marches in Bilbao and San Sebastian -- which drew the largest crowds -- and in Vitoria, all passed off peacefully.
The banner leading off the march at Bilbao declared, "Open the doors to independence and long live the free Basque Country."
Banners at the demonstration in San Sebastian were more militant, with one denouncing the ruling Socialists as terrorists. Another, written in the Basque language, read: "You will pay for what you have done."
Another, authorised demonstration in the city of Pamplona in the neighbouring region of Navarre, was broken up by police, Spanish media reported.
Earlier Saturday, there were sporadic incidents of violence in the Basque Country including the burning of a post office on Saturday, following Thursday's arrests, regional police said..
Four masked men burst into the post office in the town of Lezo, told the single employee present to leave, then sprinkled the room with petrol and set it alight, gutting it.
In other incidents, petrol bombs were thrown at a courthouse and municipal office building at Pasai San Pedro, and an attempt to set a bus on fire at Markina was thwarted by police.
The actions were typical of youthful supporters of ETA, which is fighting for independence of the Basque Country of northern Spain and southwestern France.
A police raid late Thursday on a meeting of Batasuna in the Basque town of Segura led to the arrest of 23 people. They included 19 members of the banned party's leadership, two of whom are French.
They were to appear Sunday morning before Spanish judge Baltasar Garzon, who ordered the crackdown in a hardening of the government's stance on ETA since the group officially ended a 15-month-old ceasefire in June.
Barrena denounced the detention of "23 activists from the independent left" and that it amounted to "a declaration of war" seeking to "slam the door on Basque independence."
He called for a renewed commitment "to pursue the fight for independence and socialism."
Spanish radio said some of the suspects were being transferred to Madrid Saturday, but police searches in the region were still going on.
Premises raided included a property near San Sebastian of the Communist Party of the Basque Lands, which is allied to Batasuna but not banned.
On Friday hundreds of people demonstrated in protest against the arrests in various Basque cities, including 1,000 in Bilbao, the region's financial and cultural centre.
ETA has killed 819 people during almost four decades of fighting for its cause. Both Batasuna and ETA are listed as terrorist organisations by the European Union and the US government.
Batasuna with Marxist-Leninist leanings has been banned as a party since 2003 for refusing to condemn violence and cut its links to ETA. The detained leaders face charges related to Garzon's investigation into those links, especially the party's suspected financing of ETA's activities.
Barrena said on Friday that the roundup was designed to boost the Socialist government ahead of March elections, when the question of regional autonomy and the "Basque problem" are expected to be major issues.
On Thursday Spanish Prime Minister Jose Luis Rodriguez Zapatero said: "ETA has only one option which is to lay down its arms, to abandon violence," as he rejected any dialogue with the armed group.