WikiLeaks founder Julian Assange has been found guilty of breaching his bail.
Mr Assange has appeared at a court in Westminster, and could go to prison for up to 12 months when he is sentenced.
He is also facing extradition to the US following his arrest at the Ecuadorian embassy in London, earlier almost seven years since going inside.
He was arrested by British police and carried out of the embassy after his South American hosts abruptly revoked his asylum.
Sporting a long white beard and wagging a finger, he shouted "UK must resist" as he was taken away in handcuffs by seven men and hauled into a police van.
Assange arrest video:
"The UK must resist this attempt by the Trump administration"https://t.co/fq1JGfw40u
— WikiLeaks (@wikileaks) April 11, 2019
He has been in the building for nearly seven years after seeking refuge to avoid extradition to Sweden.
WikiLeaks said on Twitter that the Ecuadorian ambassador "invited British police into the embassy" and Mr Assange was immediately arrested.
Scotland Yard said Mr Assange (47) was being held on behalf of the US authorities, as well as for breaching his original bail conditions.
Following the arrest, the US Department of Justice said he was being held in connection with a federal charge of conspiracy to commit computer intrusion for agreeing to break a password to a classified government computer.
The judge said the US must produce its case for requesting his extradition from Britain by June 12th.
Speaking to the House of Commons after Mr Assange's detention, British Prime Minister Theresa May said his arrest shows that "no one is above the law."
Mrs May's spokesperson said Britain had not lobbied for his asylum to be revoked.
The UK's Foreign Secretary Jeremy Hunt said: "Julian Assange is no hero. He's hidden from the truth for years and years, and it is right that his future should be decided in the British judicial system."
In a tweet, he thanked the government of Ecuador for its cooperation on the arrest.
Julian Assange is no hero and no one is above the law. He has hidden from the truth for years. Thank you Ecuador and President @Lenin Moreno for your cooperation with @foreignoffice to ensure Assange faces justice
— Jeremy Hunt (@Jeremy_Hunt) April 11, 2019
"Everybody wants to bring this to an end," Britain's Europe Minister Alan Duncan said.
"Anyone who's been holed up in a room for seven years is really going to suffer, mentally and physically, so just on a human level it was important that this was brought to an end."
Mr Duncan 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 in a video statement released on Twitter: "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."
Mr Moreno said he asked UK officials to guarantee that Mr Assange would not be extradited to a country where he could face torture or the death penalty.
"The British government has confirmed it in writing," Mr Moreno added.
In a sovereign decision Ecuador withdrew the asylum status to Julian Assange after his repeated violations to international conventions and daily-life protocols. #EcuadorSoberano pic.twitter.com/pZsDsYNI0B
— Lenín Moreno (@Lenin) April 11, 2019
WikiLeaks said Ecuador had acted illegally in terminating Mr Assange's political asylum "in violation of international law". His relationship with his hosts collapsed after Ecuador accused him of leaking information about Mr Moreno's personal life.
His attorney, Barry J Pollack, said it was "bitterly disappointing" that a country would allow someone to whom it has extended asylum to be arrested in its embassy.
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.
Images of Ecuador's ambassador inviting the UK's secret police into the embassy to drag a publisher of--like it or not--award-winning journalism out of the building are going to end up in the history books. Assange's critics may cheer, but this is a dark moment for press freedom. https://t.co/ys1AIdh2FP
— Edward Snowden (@Snowden) April 11, 2019
Actress Pamela Anderson, who called on world leaders to intervene to free Mr Assange while he was still at the embassy, tweeted that she was "in shock" and accused Ecuadorian, UK and US officials of complicity in his arrest.
I am in shock..
I couldn’t hear clearly what he said?
He looks very bad.
How could you Equador ?
(Because he exposed you).
How could you UK. ?
Of course - you are America’s bitch and
you need a diversion from your idiotic Brexit bullshit.
— Pamela Anderson (@pamelaanderson) April 11, 2019
Mr Assange was holding a copy of Gore Vidal's "History of the National Security State" when he was brought out of the embassy. He continued reading it in court.
The WikiLeaks founder 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 his arrest that she would push to have prosecutors re-open the probe.
His arrest comes a day after Wikileaks accused the Ecuadorian 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.