A defiant Mr Assange, sporting a long white beard and waving a finger, was carried out and hauled into a police van.
He has been in the building for nearly seven years after seeking refuge to avoid extradition to Sweden and was detained on Thursday after the Ecuadorian government withdrew his asylum.
Following Mr Assange's arrest, Europe minister Sir Alan Duncan said: "It is absolutely right that Assange will face justice in the proper way in the UK''.
"It is for the courts to decide what happens next."
Sir Alan said they are grateful to Ecuador's government under President Lenín Moreno for the action they have taken and that today's events follow "extensive dialogue between our two countries."
Mr Moreno said on Twitter: "In a sovereign decision Ecuador withdrew the asylum status to Julian Assange after his repeated violations to international conventions and daily-life protocols."
In a video statement on Twitter Moreno also said : "Today I announce that that the discourteous and aggressive behavior of Mr Julian Assange, the hostile and threatening declarations of its allied organization, against Ecuador, and especially the transgression of international treaties, have led the situation to a point where the asylum of Mr Assange is unsustainable and no longer viable."
WikiLeaks said Ecuador had acted illegally in terminating Assange's political asylum "in violation of international law".
Mr Assange was holding a copy of Gore Vidal's History of the National Security State when he was brought out of the building.
NSA whistleblower Edward Snowden, who lives in exile in Russia, said in a tweet that images of Mr Assange's arrest are going to end up in the history books.
"Assange's critics may cheer, but this is a dark moment for press freedom," Mr Snowden wrote.
The WikiLeaks founder, 47, has been living in Ecuador's London embassy since 2012 after he sought refuge there following a British judge ruling he should be extradited to Sweden to face sexual assault allegations.
Sweden dropped the case in 2017 but Mr Assange remained in the London embassy as he feared being extradited to the US to face charges over the WikiLeaks website's release of sensitive US government files.
A Swedish lawyer representing the alleged victim in a rape investigation involving Mr Assange said following the arrest she would push to have prosecutors reopen the probe.
Scotland Yard said in a statement that Mr Assange has been taken into custody at a central London police station, where he will remain, before being presented before Westminster Magistrates' Court as soon as is possible.
His arrest comes a day after Wikileaks accused the Ecuadorean Government of an "extensive spying operation" against Mr Assange.
WikiLeaks claims meetings with lawyers and a doctor inside the embassy over the past year were secretly filmed.