In Marquesina, Vizcaya, three masked men hijacked a bus, forcing the driver and two passengers to get off, and then set fire to the vehicle.
The bus was completely burned out and two nearby cars were also damaged in the attack.
The attack comes at a time when 34 suspected members of the outlawed youth organisation Segi are appearing before judge Grande-Marlaska on charges of terrorist collaboration. The 34 individuals were arrested on Tuesday.
The Supreme Court declared Segi to be a terrorist organisation and the group was outlawed.
Judge Grande-Marlaska has so far heard testimony from 13 of the accused and has jailed 11 of them. The other two, contrary to requests by the prosecutor, were released from custody, although they have to report to the court each week and may not leave the country.