An interior ministry spokesman said a person calling on behalf of Eta, a group that has fought for decades for an independent Basque state, warned police that a bomb was to explode at 10am local time (0900 GMT) at the re-transmitter.
 
Bomb attack
 
The explosion appeared to be Eta's second attack in 2008 after a bomb hidden inside a backpack caused considerable damage to a courthouse in Spain's Basque region earlier this month.
 
Jose Luis Rodriguez Zapatero, Spain's prime minister, faces criticism over failed Eta peace talks and has launched a crackdown on the Basque separatists and their allies.
 
He will be fighting to retain office in the March 9 elections.
 
Eta has killed more than 800 people during its four decade campaign.
 
The most recent deaths were in December, when Eta fighters shot dead two undercover Spanish policemen in southern France.
 
Eta prisoners
 
Also on Friday, a letter by two jailed former Eta commanders, published in a Spanish newspaper, criticised the Basque separatist group's decision to end its ceasefire with the government.
 
Jose Luis Alvarez Santacristina and Kepa Pikabea said in the letter in Basque newspaper Noticias de Guipuzcoa that, "they were not at all in agreement with the armed political strategy that is once again being followed (by Eta)".
 
"We have for a long time believed that no armed strategy will allow us to get out of the conflict," they said in the letter, which the newspaper said was written in November.
 
The publication of the letter comes just days after Basque political prisoners' group EPPK expelled both men for supporting "the strategy of the government of Spain".