The website of Spain's national police force was hacked by a group that calls itself Anonymous, BBC reported.
The attack on the site was carried out in retaliation for the arrest of three Spanish nationals who, the police claimed, were core members of the group.
The hackers managed to keep the police website offline for about an hour from 2130 GMT June 12.
Though authorities did not confirm that Anonymous was behind the attack, a statement was posted on a website linked to Anonymous that claimed responsibility for the hacking.
The group said it had used a distributed denial of service (DDoS) attack which bombards a target website with so much data that it becomes overwhelmed.
In the statement, Anonymous said the attack was a "direct response to the Friday arrests of three individuals alleged to be associated with acts of cyber civil disobedience attributed to Anonymous".
Anonymous denied that the men arrested were part of the "core" of the group.