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The country's King and Queen, along with Mexico's President Vincento Fox, were due to open a major art exhibition at the centre later today.
At least 10 people are reported to have suffered light injuries.
The Basque separatist group ETA is reported to have warned it was planning an attack.
The king and queen of Spain and Mexican President Vicente Fox were scheduled to open an art exhibition at the convention center several hours later.
Police said a Basque newspaper received a warning in the name of the separatist group, ETA, 30 minutes before the blast.