By Laurie Segall @CNNMoneyTech

NEW YORK (CNNMoney) -- In one of the U.S. government's largest anti-piracy crackdowns ever, federal agents on Thursday arrested the leaders of and shut down Megaupload.com, a popular hub for illegal file sharing.

Hours later, Megaupload's fans turned the table on the feds. "Hacktivist" collective Anonymous set its sights on the U.S. Department of Justice and apparently knocked the agency's website offline.

"We are having website having website problems, but we're not sure what it's from," a DOJ spokeswoman told CNNMoney.

The DOJ website glitches came soon after various Twitter accounts associated with Anonymous took aim at the agency.

"One thing is certain: EXPECT US! #Megaupload" read one tweet from AnonOps.

One hour later, the same account tweeted a victory message: "Tango down! http://universalmusic.com & http://www.justice.gov// #Megaupload"

Universal Music's website also went down Thursday afternoon. The music company had been locked in a legal battle with Megaupload.

The Anonymous attack came soon after the DOJ announced the indictment of seven individuals connected to Megaupload for allegedly operating an "international organized criminal enterprise responsible for massive worldwide online piracy of copyrighted works."

Authorities said the operation had generated more than $175 million in illegal profits through advertising revenue and the sale of premium memberships.

According to the indictment, Megaupload, which launched in 2005, was once the 13th most visited website on the Internet, serving as a hub for distribution of copyrighted television shows, images, computer software and video games.

The site's popular MegaVideo subsidiary was widely known in tech circles for its copious selection of pirated content, including recent movies and episodes of hit TV shows.

Four of those indicted were arrested Thursday in Auckland, New Zealand, at the request of the U.S. Three others remain at large.

The individuals indicted are citizens of New Zealand, Germany, Slovakia and the Netherlands. No U.S. citizens were named, however, Megaupload has servers in Ashburn, Virginia and Washington D.C., which prompted the Virginia-based investigation.

FBI agents executed 20 search warrants in the U.S., while authorities conducted additional searches in eight other countries. Officials say they seized about $50 million in assets.

The news comes as lawmakers have turned their attention to anti-piracy legislation. Protests erupted both online and offline this week against two bills currently under consideration in Congress: the House's Stop Online Piracy Act (SOPA) and the Senate's Protect IP Act (PIPA).

The bills are aimed at cracking down on copyright infringement by restricting access to sites that host or facilitate the trading of pirated content. But the legislation has created a divide between tech giants, who say the language is too broad, and large media companies, who say they are losing millions each year to rampant online piracy.

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-CNN Justice Producer Terry Frieden contributed to this report.