The official Web site of the Holy See was not operational Wednesday afternoon and a spokesman confirmed it had been hacked, CNN reported.
Anonymous, a loose confederation of computer hackers, claimed responsibility.
"Anonymous has now decided to lay siege to your site in response to the doctrines, liturgies and the precepts, absurd and anachronistic, that your organization, (which) is for profit, propagates and spreads worldwide," CNN said a statement in Italian on the blog "ufficiale di Anonymous Italia" read.
"This is NOT intended to attack the Christian religion or against the faithful around the world, but to the corrupt Roman Apostolic Church and all its emanations," the statement said.
Hackers also took down the home page of Panda Security's Web site, the Spanish-based software security company said.
Eweek.com said the Panda Security attack appears to be in retaliation for the arrests of several alleged members of the hacker group LulzSec.
The company's Web site was taken down Wednesday but Panda Security, in a post on its Facebook page, said the attack did not breach the company's internal network "and neither source code, update servers nor customer data was accessed."
The company said the only information accessed was related to marketing campaigns and some obsolete credentials.
"We continue investigating the cause of the intrusion and will provide more details as soon as they become available. Meanwhile we assure all our customers and partners that none of their information has been compromised and that our products and services continue functioning as normal."
Eweek.com said Anonymous apparently used the site as an opportunity to take aim at Hector Xavier Monsegur, alleged leader of the LulzSec group, whose cooperation with authorities helped lead to Tuesday's arrests of five computer hackers in the United States, Scotland, England and Ireland for computer hacking and other crimes.
Monsegur had used the online name "Sabu" in his work with LulzSec.
"Yeah yeah, we know, Sabu snitched on us," a message on the hacked PandaLabs page allegedly said. "As usually happens, FBI menaced him to take his sons away. We understand, but we were your family, too."
The hackers also promised to continue their activities and challenged the FBI and other authorities to try to find them.
"To FBI and others ... come at us bros we are waiting for you," the message read, eweek.com said.
The five hackers charged Tuesday identified themselves as aligned with the group Anonymous or offshoot groups including LulzSec, the Justice Department said.
Monsegur was secretly arrested last year and pleaded guilty on Aug. 15, 2011, in federal court to computer-hacking conspiracies and other crimes, prosecutors said.
Prosecutors allege Monsegur and other members of Anonymous are responsible for a number of cyberattacks between December 2010 and June 2011, including denial-of-service attacks against the Web sites of Visa, MasterCard and PayPal, as retaliation for the refusal of these companies to process donations to WikiLeaks, as well as hacks or denial-of-service attacks on foreign government computer systems.