MADRID, July 28, 2008 (AFP) - A small bomb placed by the separatist group ETA went off overnight at a highway building site in Spain's northern Basque region, a Basque interior ministry spokeswoman said Monday.

The explosion near the town of Orio caused no injuries but damaged four vehicles and six shacks used by work crews, the spokeswoman added.

The damage was discovered Monday morning when workers arrived at the site.

Anti-terrorism judge Baltasar Garzon on Sunday ordered the detention of seven people on charges of belonging to an ETA cell suspected of carrying out the majority of the group's attacks since it called off a ceasefire in June 2007.

All seven, including the suspected leader of the cell, Arkaitz Goikoetxea, had been arrested in police raids carried out last week.

Garzon released three others who were held as part of the same operation, two without conditions and one on bail.

ETA set off four weak bombs at two northern seaside resorts, Laredo and Noja, on July 20 which slightly injured a woman who was struck by a rock that was sent flying by one of the blasts.

The group is blamed for the deaths of 823 people in its 40-year campaign of bombings and shootings to carve a Basque homeland out of northern Spain and southwestern France.