El Mundo said the group had warned police in a statement it planned to disable the website at some time on Saturday. The website was down for at least an hour from 2130 GMT, the paper said.
A spokesman for the Spanish police said access to the website www.policia.es was blocked at 2 a.m. on Sunday, but said police were still studying the reason for this.
“A website can collapse if too many people try to access it at once. I cannot confirm the link with the Anonymous group,” the spokesman said.
Spanish police arrested three suspected members of the group on Friday on charges of cyber attacks against targets including the websites of Sony Corp, governments, businesses and banks — but not the massive hacking of PlayStation gamers.
Anonymous responded by threatening to retaliate for the arrests: “We are Legion, so EXPECT US,” the group said on its official Twitter feed.
Access was also temporarily blocked to the websites of Spain’s state employment agencies INEM and SEPE, El Mundo said, but cited the Labor Ministry as saying this was due to maintenance problems and not related to a cyber-attack.
Anonymous is a loose grouping of activists who frequently try to shut down the websites of businesses and other organizations that it opposes.
Members cripple websites by overwhelming them with traffic in what is commonly known as “denial of service” attacks.
The group has attacked Turkish government websites in a protest against Internet censorship.
In a separate development, the International Monetary Fund became the latest known target of a major cyber attack on Saturday.
Jeff Moss, a self-described computer hacker and member of the Department of Homeland Security Advisory Committee, said he believed that attack was conducted on behalf of a nation state looking to either steal sensitive information about key IMF strategies or embarrass the organization to undermine its clout.