PAMPLONA, Spain (CNN) -- Fierce clashes between opposing protesters erupted Sunday in northern Spain hours after hospital officials announced the death of a young politician held captive by Basque separatists.
The slaying also prompted the pope to condemn the act Sunday, calling it a "barbarous assassination." The Spanish government in its first official statement called the killing an act of "unjustified barbarity."
City councilman Miguel Angel Blanco , 29, died in a hospital early Sunday about 12 hours after his body was found dumped on a country road with two gunshot wounds to the head.
Blanco had been held captive for two days by the armed militant group ETA, which had warned it would kill him unless the government transferred Basque prisoners to jails in the Basque region of northern Spain.
In the northern city of Pamplona , hundreds of young Spaniards shouting "Sons of bitches!" and "Murderers!" clashed with ETA sympathizers outside the offices of the group's political wing, Herri Batasuna.
The rival groups threw bottles and rocks as police in riot gear tried to break up the clashes.
When demonstrators tried to storm Herri Batasuna's headquarters, police sprayed tear gas and fired rubber bullets into the crowd. A number of people were bleeding and bruised in the clashes, witnesses said.
Pamplona was packed with tens of thousands of visitors for the running of the bulls -- Spain's most popular festival -- when the clashes erupted. Most of the rioting youths were dressed in the traditional fiesta white costume with red headbands and sashes.
The festival was suspended from Saturday night to Sunday night to show "sadness and indignation" over the killing.
Herri Batasuna's offices in Blanco's hometown of Ermau were attacked with firebombs late Saturday, causing some damage but no injuries.
The kidnapping and shooting stirred emotions in Spain unlike any other event in recent memory.
"The gravity of this brutal terrorist act degrades its instigators, its authors and anyone who supports them in any form," Bishop Ricardo Blazquez of the northern city of Bilboa said in a statement Sunday.
ETA had warned it would kill Blanco by 4 p.m. Saturday unless its demands were met.
Within an hour of the deadline, people who heard gunshots in a rural area found Blanco -- bound and shot twice in the head. Soon, spontaneous anti-ETA demonstrations broke out in Madrid, Barcelona, Zaragoza and other cities.
A trained accountant, Blanco was a low-ranking official for the ruling conservative Popular Party in Ermau and was a minor political figure until the kidnapping. The reason he was targeted was unclear.
The front pages of Spain's Sunday newspapers reflected the country's angry mood. "Sons of Bitches," declared a headline in Diario 16. "Beasts!" read another in the newspaper YA.
Pope John Paul II denounced the killing as an "act of blood."
"I strongly deplore this bloody act. The killing of an innocent person can never be justified," he said.
The pope also said he was praying that the "beloved Spanish people" will have the courage and strength for "coexistence in peace and serenity."
The pope made his comments from a balcony at his vacation villa in the Italian Alps near France. Also Sunday a Basque-region pacifist group, Gesture For Peace, urged people to be calm.
Spanish Interior Ministry spokesman Cayetano Gonzalez issued the government's first official reaction. He said Herri Batasuna would suffer the consequences of the killing because Spaniards do "not tolerate people who do not condemn this type of unjustified barbarity."
The kidnapping ordeal, which was also condemned by France, touched a raw nerve among Spaniards, who in recent months have faced a growing wave of violence by the rebels. ETA, a Basque language acronym for Basque Homeland and Freedom, has killed nearly 800 people in its 29-year fight for independence.
More than a million Spaniards showed their outrage at the kidnapping in a series of massive peaceful demonstrations across the country on Friday and Saturday.
Blanco died at a San Sebastian hospital with his mother, sister and girlfriend nearby after being admitted in a coma, Dr. Francisco Garcia Urra told reporters.
"The wounds were very deep," he said. "It was totally impossible to save him."
Blanco's shooting came just 11 days after police rescued a prison official who had been held hostage by ETA for 1 1/2 years to press its demand that ETA inmates be transferred to jails in the Basque region.
© 1997 Cable News Network, Inc.