Hacktivists affiliated with the "Anonymous" movement took credit for crashing the websites of the U.S. Department of Justice and the Universal Music Group on Thursday after the FBI shut down one of the most popular file sharing websites, MegaUpload.com.

"The DOJ web server hosting justice.gov is experiencing a significant increase in activity, resulting in a degradation in service," the Justice Department tweeted hours after going offline. "The department is working to ensure the website is available while we investigate the origins of this activity which is being treated as a malicious act until we can fully identify the root cause of the disruption."

The group later took credit for crashing the site of the Recording Industry Association of America and the Motion Picture Association of America. They also briefly knocked the sites of the U.S Copyright Office site, the record label BMI and the French copyright authority HADOPI offline. "Anonymous" hackers also gained internal access to the Utah Chiefs Of Police Association site.

The sites were apparently subjected to a distributed denial of service attack (DDoS), where the bandwidth or resources of a web server is flooded with Internet traffic in hopes of knocking it offline.

MegaUpload, which had more than 150 million registered users, was shut down by the FBI because of alleged copyright infringement despite the fact that it claimed to comply with copyright infringement notices. The site allowed registered users to upload files, which could then be downloaded by others.

The owners of the site claimed to work with copyright enforcement agencies to remove infringing material and said they complied with Digital Millennium Copyright Act (DMCA) notices. But prosecutors said they were not diligent enough and labelled the site a "mega conspiracy," which cost copyright holders $500 million.

"In exchange for payment, the Mega Conspiracy provides fast reproduction and distribution of infringing copies of copyrighted works from its servers located around the world," the indictment said.

Seven of its founders and employees have been charged with online piracy crimes. Four of those suspects have already been arrested in New Zealand, according to the Wall Street Journal.

The charges include copyright infringement as well as conspiracies to commit racketeering, copyright infringement and money laundering.

The shut down of MegaUpload came after the largest online protest in history. Thousands of sites joined in a blackout protest on Wednesday against the House’s Stop Online Piracy Act (SOPA) and the Senate’s PROTECT IP Act (PIPA). Some sites, like Wikipedia, completely shut down and displayed only message urging people to oppose the bills. Other sites, like Google, continued functioning normally but put up a black background and predominately displayed their opposition to the bills.

The author of PIPA praised the FBI's actions on Thursday.

“Today’s action by the Department of Justice against the leaders of MegaUpload.com shows what law enforcement can do to protect American intellectual property that is stolen through domestic websites," Sen. Patrick Leahy (D-VT) said in a statement. "Unfortunately, there are no tools in the arsenal to protect that same American intellectual property from theft by websites hosted and operated overseas."

"Why should we give greater protections to criminals engaging in the same conduct overseas?" he continued. "Meaningful legislation to stop online infringement and piracy by foreign rogue websites will protect American workers, American consumers and America’s economy. The PROTECT IP Act would close this gap and offer a meaningful solution to this costly and corrosive problem.”

Updated at 7:23pm ET.