Spain's Princess Letizia and Prince Felipe comfort relatives of the police killed in an explosion in Mallorca

The leaders of Spain's main political parties have paid tribute to two police officers killed in an explosion as security forces guarded against a fresh attack on the 50th anniversary of the founding of the armed Basque group Eta.

The interior ministry's representative in Mallorca said police were working on the hypothesis that terrorists were holed up in an apartment on the island waiting for the situation to cool down. The ministry issued photographs of six suspected Eta members.

Authorities blame Eta for an explosion that killed the two policemen in Mallorca on Thursday and a car bomb that wounded more than 60 in Burgos on Wednesday.