WikiLeaks supporters calling themselves “hacktivists” may target Twitter next after successfully bringing down websites belonging to Visa, MasterCard and the Swedish government and Sarah Palin.

The group of hackers – “hacktivists” – call themselves Anonymous and label their campaign “Operation Payback”. The group has been using Twitter and Facebook to coordinate attacks on target sites. Both social networks have now removed the group.

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Anonymous brought down sites belonging to Visa and MasterCard after the credit card companies refused to process donations to Wikileaks.

The hackers also infiltrated the Swedish government’s website, taking it offline for more than 12 hours. Wikileaks founder Julian Assange has been arrested after the Swedish court issued a warrant for sex crimes.

U.S. Tea Party frontwoman Sarah Palin had her website hacked, apparently by Wikileaks supporters, after she criticised Assange.

“This is what happens when you exercise the First Amendment and speak against his sick, un-American espionage efforts,” Palin told ABC News.

Will Twitter be next?

Anonymous may hit Twitter next after the social networking site suspended AnonOps’ account last night. Twitter has also been accused of preventing the term “Wikileaks” from appearing in its trending topics – although it has denied this.

AnonOps has posted a statement on its website warning: “Twitter you’re next for censoring WikiLeaks discussion.”

In a blog linked to its Twitter account, Anonymous wrote: “The internet is the last bastion of freedom in this evolving technical world. The internet is capable of connecting us all. When we are connected we are strong.

When we are strong we have power. When we have power we are able to do the impossible. This is why the government is moving on WikiLeaks. This is what they fear. They fear our power when we unite. Do not forget this.”