MADRID (Reuters) - A bomb exploded overnight in a shop owned by a Basque town councillor, capping a weekend of attacks in northern Spain in retaliation for the arrests of suspected ETA separatist guerrillas, officials said on Monday. Spanish authorities blamed the violence on radical youth groups linked to ETA, which declared a unilateral ceasefire last September in its 30-year fight for an independent Basque homeland. An explosion tore through a furniture business in Erandio owned by a local politician of Spain‘s ruling centre-right Popular Party, police said. The shop, empty at the time, was seriously damaged by the homemade bomb, but no one was injured. It was the latest in a series of weekend attacks in the Basque country in response to a joint crackdown on ETA by Spanish and French police that led to the arrest of the guerrillas‘ suspected military chief in Paris. Police defused a bomb planted outside the family home of a Socialist candidate for mayor of Bilbao. Another explosive device was detonated outside the law office of a Socialist town councillor in Ordizia, causing minor damage but no injuries. Masked youths threw petrol bombs at a car dealership in Bilbao, burning several vehicles. Spanish Interior Minister Jaime Mayor Oreja condemned the attacks and said they were part of an "organised strategy" to undermine moves towards peace. ETA has honoured its ceasefire without incident since it began nearly six months ago, but authorities have become alarmed by an escalating campaign of street violence and threats against anti-separatist politicians.