|Eta claims the Rioja province is part a Basque homeland they aim to secure [AFP]|
State radio said that following the blast the Civil Guard told local residents to stay indoors, indicating that another bomb was feared to be in the vicinity.
Benjamin Jones, a journalist speaking from Madrid, told Al Jazeera that it was fortunate there were not many injuries, as the area was very crowded.
"A Good Friday procession had just finished in the area, but the police were able to evacuate everyone before the blast went off."
"Eta has previously staged attacks on days of religious or political significance, and that certainly happened again today."
The province of Rioja lies across the river Ebro from Navarra, which is not administratively part of the Basque region but is claimed by Eta to be Basque.
Eta aims to create an independent Basque homeland in northern Spain and southwestern France.
A ceasefire agreement with the Spanish government ended in 2006, after peace talks failed.
The group most recently claimed responsibility for the killing of a former town councillor in the Basque region two days before the Spanish general elections on March 9.
More than 800 people have been killed since the late 1960s in Eta's battle for independence.
Source: Al Jazeera and agencies