Tens of thousands of Basques took to the streets Sunday in the largest demonstration by the ETA guerrillas' political wing since the killing of a politician by separatist rebels earlier this month.
With Spain still seething over the murder of 29-year-old Miguel Angel Blanco, Basque authorities bolstered security at the pro-ETA march with hundreds of police and private security guards.
The demonstration ended peacefully, but sporadic violence later broke out after 30 hooded people clashed with police.
Uniformed police, who had kept a low profile during the march earlier in the day, were deployed onto the streets of the old quarter of the northern Spanish city and fired rubber bullets in the air.
Herri Batasuna, the ETA's political arm, called the protest in response to anti-ETA rallies held across Spain in the past two weeks that drew about 6 million people.
The Basque regional government had barred a similar demonstration planned for July 19 because of fears of a possible violent backlash by people outraged by Blanco's killing. Guerrillas kidnapped the Basque town councilor July 10 and shot him in the head 48 hours later.