It was the latest in a series of retaliatory attacks on the Web sites of multinational companies and other organizations that Internet activists deemed hostile to the WikiLeaks anti-secrecy organization and its jailed founder, Julian Assange, The New York Times reported.
Earlier, Internet activists brought down the Swedish government's Web site and threatened more attacks in an escalating cyber battle.
The official Swedish site was offline for several hours overnight with a message saying the site could not be accessed, Britain's Daily Telegraph said.
Swedish prosecutors issued an international arrest warrant against Assange on sexual assault allegations.
Commercial Web sites, including Visa and MasterCard, already had been blitzed after saying they no longer would do business with WikiLeaks, the whistle-blowing Web site that recently made available more than 250,000 U.S. State Department cables, some of which were included in several newspaper articles.
A group called "Anonymous" claimed responsibility for the attacks, saying they were carrying out "Operation Payback."
Dutch authorities said they arrested a 16-year-old boy suspected of being involved in attacks on the Web sites of MasterCard and Visa, The Wall Street Journal reported.
Assange was in a London jail after he surrendered to authorities earlier this week. The charges filed by Swedish prosecutors are not related to WikiLeaks, and Assange said the sexual assault allegations are part of a smear campaign.
A 22-year-old software engineer in London who goes by the moniker "Coldblood" said the cyberattacks by Anonymous aren't over, several British publications reported. Actions so far have been a "distributed denial of service," in which the target site is hit with a huge number of visitors intent on forcing the site to exceed its capacity and crash.
"The campaign is not over, it's still going strong. More and more people are joining," the Telegraph reported Coldblood as saying. "I see this as becoming a war -- but not a traditional war. This is a war of data. We are trying to keep the Internet free for everyone."
Coldblood said Web sites seen as "bowing down to government pressure" are targets, The Daily Mail reported.
"We feel that WikiLeaks has become more than just about leaking of documents, it has become a war ground, the people versus the government," Coldblood said. "The idea is not to wipe them off but to give the companies a wake-up call."
Anonymous has been tied to attacks on Web sites belonging to the Church of Scientology and the music industry.