No group has admitted carrying out the attack.
Suitcase warning
Police discovered a suitcase an hour before the bomb exploded, with the words "Beware Bomb" written on it, a police spokesman said.
The blast came a day after Spain's Socialist-dominated parliament was inaugurated.
Police said the 10 people were from a group suspected of carrying out violence against police, banks, railway stations and public transport.
They mentioned, in particular, arson attacks on a courthouse, the car of a Socialist politician and a government building in three separate incidents in Renteria in 2005.
Acts of urban violence, usually involving Molotov cocktails, are a regular occurrence in the Basque and Navarra regions.
Police blame young Basques belonging to groups with links to Eta.
But the attacks have become more common since Eta declared an end to a 15-month-old ceasefire last June, and the government launched a crackdown on the group and its political wing, Batasuna.
Eta has killed more than 820 people since the late 1960s in its campaign for an independent Basque country in northern Spain and southwestern France.