An emergency services official said 23 of the injured were taken to hospital on Wednesday, though none were in a critical condition.
An anonymous caller claiming to represent ETA alerted the Basque newspaper Gara half an hour before the blast, a police spokesman said.
Spanish King Juan Carlos had been due later in the day to open the Arco contemporary art fair at the conference centre with visiting Mexican President Vicente Fox.
ETA has been blamed for the deaths of more than 800 people in its three-decade-plus campaign for an independent Basque homeland straddling the Pyrenees between France and Spain.
The organisation's last attack was on 30 January, when a blast in a hotel on Spain's Costa Blanca slightly injured one person.
Several people suspected of links with the banned group were arrested on Wednesday in a sweep in the Basque country in northern Spain, local authorities said.
Spain's King Juan Carlos was due
to visit the conference centre
"The number of people arrested and the exact location of the arrests cannot be divulged for the time being because the operation is continuing," an official said.
Spanish news agencies quoted the regional interior ministry as saying the operation during the night had provoked a stand-off between regional police and a group of people who had blocked a road with containers in a coastal town.
In March 2003, 191 people were killed and hundreds injured when 10 bombs exploded on four trains in three stations in Madrid, days before a general election.
The Spanish government has blamed al-Qaida groups in Europe for the attacks.