MADRID, Spain — A senior member of the Basque separatist movement said Spain’s government had effectively declared war by arresting most of his outlawed party’s leadership while thousands demonstrated Saturday to demand independence for the northern region.

Police detained 23 leaders of the Batasuna party — considered the political wing of the armed militant group ETA — while they held a secret meeting Thursday in the town of Segura.

“This police operation uncovers the decision of the Spanish government, which is to issue a total declaration of war against Basque independence,” Pernando Barrena told a news conference in the city of San Sebastian in the Basque region.

Prime Minister Jose Luis Rodriguez Zapatero had pursued a peace process with ETA after it declared a cease-fire in March 2006. But the talks went nowhere, and ETA shattered its cease-fire with a December bombing that killed two people. The government has since arrested dozens of ETA suspects and is now going after their political allies.

Batasuna’s most prominent leader, Arnaldo Otegi, was jailed on terrorism charges in July.

Barrena, who apparently was not at Thursday’s secret meeting, is the only Batasuna leader who has not been detained. He said the raid was revenge for the movement’s hard-line position in the peace talks and called the arrests “kidnappings.”

Thousands of Batasuna supporters marched in several Basque cities to protest the arrests and call for the region’s independence. Media reports said protests in the cities of Bilbao and San Sebastian each drew 3,000 people. Police declined to give a figure.

In the city of Pamplona, a march of 300 people ended in clashes between truncheon-wielding police and masked youths. Several people were injured, police said.

Batasuna was banned in 2003 in a Supreme Court ruling that barred it from running in elections and holding formal meetings. The party, however, has often been allowed to hold news conferences and stage street rallies. Zapatero had seen Batasuna as a potential bridge to ETA militants in the peace process.

Anti-terror judge Baltasar Garzon, who ordered Thursday’s raid, was expected to question the 23 detained Batasuna members Sunday. They were transferred to a Madrid prison on Saturday.

ETA has killed more than 800 people since the late 1960s in a campaign of shootings and bombings. The group seeks an independent Basque homeland in northern Spain and southwest France.