Police arrested Assange on Thursday after the South American nation revoked the political asylum that had given him sanctuary for almost seven years. Ecuador's President Lenin Moreno said he took the action due to "repeated violations to international conventions and daily-life protocols."
In Washington, the US Justice Department accused Assange of conspiring with former US Army intelligence analyst Chelsea Manning to break into a classified government computer at the Pentagon. The charge was announced after Assange was taken into custody.
His lawyer said the 47-year-old Australian would fight extradition to the United States.
Assange took refuge in the embassy in 2012 after he was released on bail in Britain while facing extradition to Sweden on sexual assault allegations that have since been dropped. He refused to leave the embassy, fearing arrest and extradition to the US for publishing classified military and diplomatic cables through WikiLeaks.
Manning, who served several years in prison for leaking troves of classified documents before her sentence was commuted by then-President Barack Obama, is again in custody in Virginia for refusing to testify before a grand jury investigating WikiLeaks.
Over the years, Assange used Ecuador's embassy as a staging post to keep his name before the public, frequently making appearances on its tiny balcony, posing for pictures and reading statements. Even his cat became famous.
But his presence was an embarrassment to United Kingdom authorities, who for years kept a police presence around the clock outside the embassy, costing taxpayers millions in police overtime. Such surveillance was removed in 2015, but the embassy remained a focal point for his activities.
Video posted online by Ruptly, a news service of Russia Today, showed several men in suits pulling a handcuffed Assange out of the embassy and loading him into a police van while uniformed British police formed a passageway. Assange, who shouted and gestured as he was removed, sported a full beard and slicked-back gray hair.
He later appeared in Westminster Magistrates' Court, where District Judge Michael Snow wasted no time in finding him guilty of breaching his bail conditions, flatly rejecting his assertion that he had not had a fair hearing and a reasonable excuse for not appearing.
"Mr Assange's behavior is that of a narcissist who cannot get beyond his own selfish interests," Snow said. "He hasn't come close to establishing 'reasonable excuse.'"
Assange waved to the packed public gallery as he was taken to the cells. His next appearance was set for May 2 via prison video-link in relation to the extradition case.
Assange's attorney Jennifer Robinson said he will fight any extradition to the US.
"This sets a dangerous precedent for all journalist and media organisations in Europe and around the world," she said. "This precedent means that any journalist can be extradited for prosecution in the United States for having published truthful information about the United States."
Asked at the White House about the arrest, President Donald Trump declared: "It's not my thing," and "I know nothing about WikiLeaks," despite praising the anti-secrecy organisation dozens of times during his 2016 campaign.
Speaking in Parliament, British Prime Minister Theresa May said the arrest shows that "no one is above the law."
Moreno said in a video posted on Twitter that Ecuador was no longer willing to give Assange protection. Other Ecuadorian officials in Quito accused supporters of WikiLeaks and two Russian hackers of trying to destabilise the country.
"The discourteous and aggressive behavior of Mr Julian Assange, the hostile and threatening declarations of its allied organisation, 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," Moreno said.
Interior Minister Maria Paula Romo said Assange's mental and physical health worsened while he was holed up, and he began to act out in aggressive ways including by smearing faeces on the walls of the embassy.
WikiLeaks quickly drew attention to US interest in Assange and said that Ecuador had illegally terminated Assange's political asylum "in violation of international law."
"Powerful actors, including CIA, are engaged in a sophisticated effort to de-humanise, de-legitimize and imprison him," the group said in a tweet over a photo of Assange's smiling face.
Moreno appeared to suggest a swift extradition to the US was unlikely.
"In line with our strong commitment to human rights and international law, I requested Great Britain to guarantee that Mr. Assange would not be extradited to a country where he could face torture or the death penalty," Moreno said. "The British government has confirmed it in writing, in accordance with its own rules."
Assange's arrest came a day after WikiLeaks accused the Ecuador's government of an "extensive spying operation" against him. It alleges that meetings with lawyers and a doctor in the embassy over the past year were secretly filmed.
In Quito, Ecuador's government denounced what it called attempts by supporters of WikiLeaks and two Russian hackers to destabilise the country as the standoff with Assange intensified recently.
Romo said a close collaborator of WikiLeaks had travelled with former Foreign Minister Ricardo Patino this year to several countries - including Peru, Spain and Venezuela - to try to undermine the Ecuadorian government. She did not identify the person.
But former Ecuadorian President Rafael Correa called Moreno's decision was "cowardly," accusing him of retaliating against Assange for WikiLeaks spreading allegations about an offshore bank account purportedly linked to Moreno's family and friends.
Edward Snowden, the former security contractor who leaked classified information about US surveillance programmes, called Assange's arrest a blow to media freedom.
"Images of Ecuador's ambassador inviting the U.K.'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," Snowden tweeted from Russia, which has granted him permission to stay there while he is wanted by the US "Assange's critics may cheer, but this is a dark moment for press freedom."
Dmitry Peskov, a spokesman for Russian President Vladimir Putin, said he could not comment on the overall case but added: "We, of course, hope that all of his rights will be observed."
An independent United Nations human rights expert said he would not halt efforts to determine whether Assange's privacy was violated at the embassy. Joe Cannataci, the special rapporteur on privacy, had planned to travel to London on April 25 to meet with Assange and said he still plans to do so - even if in a police station.
