By David Ingram

WASHINGTON (Reuters) - The United States brought criminal charges against 13 suspected members of the hacking group Anonymous on Thursday for allegedly attacking government, credit card and lobbying websites in a campaign in support of internet file-sharing.

A grand jury indictment of the 13 people was filed in U.S. District Court in Alexandria, Virginia, charging them with conspiracy to intentionally cause damage to protected computers as part of Anonymous' "Operation Payback."

The loose-knit international group known as Anonymous has been in frequent battle with U.S. authorities, not only over file-sharing but also other ideological causes such as the willingness of financial institutions to process donations for the anti-secrecy group WikiLeaks.

In March 2012, U.S. prosecutors in New York charged six suspected leaders of Anonymous for wreaking havoc on government and corporate websites.

The hackers launched "Operation Payback" in retaliation for the 2010 shutdown of Pirate Bay, a Swedish internet service that allowed users to share files such as films and music, according to Thursday's indictment.

They used what are known as denial-of-service attacks to overwhelm websites and make them inaccessible, starting with the website of the U.S. film industry lobbying group, the Motion Picture Association of America, the indictment said.

"This will be a calm, coordinated display of blood. We will not be merciful," said one set of instructions for the attacks quoted in the indictment.

Other websites targeted were those of the Library of Congress, Bank of America Corp, Visa Inc and MasterCard Inc, the Justice Department said.

Those charged ranged in age from 21 to 65 and lived in 13 different U.S. states.

(Reporting by David Ingram; Editing by Sandra Maler and Paul Simao)