UK police confirmed Thursday that Assange has been arrested "on behalf of the United States authorities" who are seeking his extradition.
He was initially arrested this morning for skipping out on a UK arrest warrant issued in 2012; once he arrived at the police station this morning, he was then "further" arrested under an extradition request by the US.
Here's the full police statement:
Julian Assange, 47, has today, Thursday 11 April, been further arrested on behalf of the United States authorities, at 10:53hrs after his arrival at a central London police station. This is an extradition warrant under Section 73 of the Extradition Act. He will appear in custody at Westminster Magistrates' Court as soon as possible.