The WikiLeaks founder was brought before Westminster Magistrates’ Court this afternoon where he was found guilty of breaching bail conditions in 2012.
Assange faces up to 12 months in prison on the charge.
He will next appear at Westminster Magistrates’ Court on 2 May by video-link in relation to the extradition case.
Assange waved to the public gallery as he was taken down to the cells.
Police detained the 47-year-old after the Ecuadorian government withdrew his asylum, blaming his “repeated violations” of “international conventions and daily-life protocols”.
Scotland Yard said in a statement that Assange was held for failing to appear in court in June 2012 and “further arrested on behalf of the United States authorities, at 10.53am after his arrival at a central London police station”.
In a statement, the British Home Office said: “We can confirm that Julian Assange was arrested in relation to a provisional extradition request from the United States of America.
“He is accused in the United States of America computer related offences.”
In a tweet, Assange’s lawyer Jennifer Robinson also said he had been “arrested not just for breach of bail conditions but also in relation to a US extradition request”.