Police officers detained the leaders of Batasuna on Thursday night as they left a secret meeting in Segura, in the Basque region of northern Spain, an Interior Ministry official and a member of Batasuna said. Television images showed hooded police officers and plainclothes officials taking handcuffed members of Batasuna to unmarked cars.

Baltasar Garzón, a high-profile antiterrorism judge, ordered the arrests, spokeswomen for the National Court and for the police said Friday.

The party leaders were accused of holding an illegal political meeting. Jone Goirizelaia, a lawyer for Batasuna, said they were being held incommunicado and she would learn the charges against each on Sunday.

The arrests are likely to bolster the government of Prime Minister José Luis Rodríguez Zapatero among voters looking for a strong stance against violent Basque separatists.

“Zapatero needs to appear as hard as the right is,” Jonan Fernández, a former member of Batasuna who now runs Baketik, a peace institute based in the Basque region, said by telephone. “The people who will pay for this are the Basques,” he said, lamenting the likelihood that progress toward ending four decades of violence would suffer.

Mr. Zapatero was criticized after ETA bombed the Madrid airport in December, killing two people and shattering a peace effort that he had praised less than 24 hours earlier. ETA insisted then that its nine-month-old cease-fire was intact, but in June declared it over.

Since then, the judiciary and the police have clamped down on ETA, considered a terrorist organization by the State Department and the European Union.

Batasuna, though banned in 2003, was given some latitude during the peace effort with ETA, when it worked with both the armed group and the government. But it faced mounting pressure after talks collapsed and it failed to condemn violence after the December bombing.

Two political activists in contact with Batasuna leaders predicted that the crackdown would actually galvanize the nationalists and help heal rifts within the party over the question of the Basque region’s political future.

Some older members of Batasuna were more ready for compromise with the Spanish government, they said, while younger ones favored a harder line. ETA says it is fighting for an independent Basque nation encompassing the Spanish and French Basque-speaking territories.

The newspaper El País said that the meeting in Segura was to allow the old guard of Batasuna to transfer power to younger leaders. Ms. Goirizelaia, the Batasuna lawyer, could not confirm this. Miren Azkarate, a spokeswoman for the Basque regional government, a nationalist coalition, said the arrests would fuel support for radicals.

“By closing all avenues to dialogue, you make a hard-line response the only option,” she said in a telephone interview.

Another Batasuna leader, Joseba Álvarez, was arrested on Tuesday on charges of organizing an illegal demonstration.