The intrusion follows hacks into the Twitter accounts of Burger King and Jeep in recent days and was claimed to be the work of the Rustle League.
Anonymous, a hacker collective which regularly targets governments and has successfully breached US government systems, has 160,000 followers of its @Anon_Central account.
Graham Cluley, a Senior Consultant at security firm Sophos, told the BBC: “The reason Anonymous fell victim is probably human weakness.
“Chances are that they followed poor password practices, like using the same password in multiple places or choosing a password that was easy to crack.
“Everyone should learn better password security from incidents like this - if it can happen to an account run by Anonymous supporters, it could happen to you."
A tweet from the anonymous account yesterday afternoon said “We’re back in business!” and the collective have seemingly began a discussion with @RustleLeague on Twitter concerning cyber security since.