Manuel Gonzalez Gonzalez was released from a penitentiary just west of Barcelona where he had been serving a 169-year sentence delivered in 1997 for horrific sexual assaults on 16 women and the resulting death of one of the victims.
"According to the experts, he is not cured of his urges," a regional interior ministry official, Ramon Espadaler, told Catalunya Radio.
Consequently, police have been ordered to keep Gonzalez under watch, Espadaler said.
However, he noted, Gonzalez "is a citizen and he is freed… he does not have a police officer by his side".
Spanish media quoted a prison medical evaluation saying Gonzalez presented a "high risk" of reoffending, and "other acts of violence" cannot be excluded.
Several of Gonzalez's traumatised victims were collectively informed by officials of his release.
Gonzalez was jailed for a 1991-1993 string of violent attacks on randomly chosen girls and women walking in Barcelona's streets in which he approached them from behind and stabbed their genitalia.
His release complied with an European Court of Human Rights ruling that found Spain had acted illegally by denying certain prisoners shortened sentences for good behaviour.
That ruling, which has created fury in Spain, has already led to the release of dozens of dangerous convicts — including rapists and members of the Basque armed separatist group ETA.