Several thousand mourners in Spain have demonstrated against ETA violence tonight after attending the funeral of a policeman slain in an attack blamed on the Basque separatist group. Representatives of Spain's mainstream political parties took part in the protest near the city of Bilbao. The 25-year-old police officer was killed in a car bomb blast that also injured his colleague near the main Basque city of San Sebastian shortly after midnight yesterday. Spanish police arrested three people shortly after the blast.