The WikiLeaks' founder was wheeled out of the embassy by the police and taken into custody in Central London on Thursday morning.
The police executed the warrant issued by the Westminster Magistrates' Court and were invited into the Ecuadorian embassy by the Ambassador.
Ecuador's president Lenin Moreno said on Twitter: "In a sovereign decision Ecuador withdrew the asylum status to Julian Assange after his repeated violations to global conventions and daily-life protocols".
The indictment said Assange in March 2010 engaged in a conspiracy to assist former US Army intelligence analyst Chelsea Manning in cracking a password stored on US Department of Defense computers connected to the Secret Internet Protocol Network (SIPRNet), a US government network used for classified documents and communications.
US prosecutors have not brought formal charges against Assange, but the WikiLeaks co-founder has suspected that he faces criminal charges under seal, meaning they have been prepared secretly.
The 47-year-old has been in the Ecuadorian embassy since 2012 when he sought to escape questioning in a Swedish sexual-assault case.
Sweden's chief Prosecutor Ingrid Isgren said she was unaware as to why Assange was arrested.
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In this February 5, 2016, file photo, WikiLeaks founder Julian Assange stands on the balcony of the Ecuadorean Embassy in London.
The group, which rose to prominence by publishing leaked United States intelligence documents, said that "very reliable sources" had told them Ecuador was last week preparing to kick Assange out of its London embassy, where he was granted refuge by previous president Rafael Correa in 2012. On Wednesday, WikiLeaks officials held a press conference where they accused Ecuador of spying on Mr Assange.
Smith told The Current's Anna Maria Tremonti that he was also not surprised by the arrest, but was "certainly saddened" by what he called "a very bad day for us all". "No one is above the law", United Kingdom home secretary Sajid Javid said in a Twitter statement.
The Ecuadorian president, Lenin Moreno, said the country had taken the "sovereign decision" to remove asylum for Assange.
WikiLeaks on Wednesday had claimed that it was being blackmailed by "dubious characters" who had obtained security camera footage of Assange inside the embassy.
Moreno specifically cited WikiLeaks' release of a batch of Vatican documents earlier this year, before listing a number of activities at the embassy in London that he did not agree with.
UK Foreign Secretary Jeremy Hunt thanked Ecuador for their "cooperation".