Hacker group Anonymous isn’t happy about the takedown of file-sharing site Megaupload, and as a result, it’s targeting some big companies and government agencies.

Earlier this afternoon, interspersed with a stream of anti-SOPA and PIPA tweets, the main Anonymous Twitter account declared, “The government takes down #Megaupload? 15 minutes later #Anonymous takes down government & record label sites. #ExpectUs.” Specifically, the group claimed responsibility for taking down the Universal Music, RIAA (the record industry’s lobbying arm), MPAA (the movie industry’s lobbying arm), and Department of Justice websites, among others. As of 3pm Pacific, the sites were still down for me, although some comments on Twitter suggested that they were returning online sporadically.

The group also claimed that the current attacks were “the largest attack ever by Anonymous,” with 5,635 participants. And it looks like the campaign is ongoing — Anonymous says it’s going after the FBI’s website next: “Get some popcorn… it’s going to be a long lulzy night.”