Mr Assange took refuge in the embassy seven years ago to avoid extradition to Sweden over a sexual assault case that has since been dropped.
The Met Police said he was arrested for failing to surrender to the court and following a US extradition request.
Ecuador's president said it withdrew his asylum after repeated violations of international conventions.
But Wikileaks tweeted that Ecuador had acted illegally in terminating Mr Assange's political asylum "in violation of international law".
Home Secretary Sajid Javid tweeted: "I can confirm Julian Assange is now in police custody and rightly facing justice in the UK."
Mr Assange, an Australian national, set up Wikileaks in 2006 with the aim of obtaining and publishing confidential documents and images. The organisation hit the headlines four years later when it released footage of US soldiers killing civilians from a helicopter in Iraq.
Former US intelligence analyst Chelsea Manning was arrested in 2010 for disclosing more than 700,000 confidential documents, videos and diplomatic cables to the anti-secrecy website.
She said she only did so to spark debates about foreign policy, but US officials said the leak put lives at risk.
Mr Assange, 47, had been in the Ecuadorian embassy in London since 2012, after seeking asylum there to avoid extradition to Sweden on a rape allegation - which he denied and was later dropped.
But he still faces a lesser charge of skipping bail in 2012 and he says this could lead to an extradition to the US for publishing US secrets on the Wikileaks website.
Scotland Yard said it was invited into the embassy by the ambassador, following the Ecuadorian government's withdrawal of asylum.
After his arrest for failing to surrender to the court, police said he had been further arrested on behalf of US authorities under an extradition warrant.
The US Department of Justice said in a statement that the extradition was in connection with federal charges of conspiracy to commit computer intrusion, relating to the Chelsea Manning revelations. They carry a maximum penalty of five years in prison.
Mr Assange was initially taken to a central London police station and is due to appear at Westminster Magistrates' Court shortly.
Ecuador 'reached its limit'This man is a son, a father, a brother. He has won dozens of journalism awards. He's been nominated for the Nobel Peace Prize every year since 2010. Powerful actors, including CIA, are engaged in a sophisticated effort to dehumanise, delegitimize and imprison him. #ProtectJulian pic.twitter.com/dVBf1EcMa5— WikiLeaks (@wikileaks) April 11, 2019
Ecuadorian president Lenin Moreno said the country had "reached its limit on the behaviour of Mr Assange" after he intervened in the internal affairs of other states.
Mr Moreno said: "The most recent incident occurred in January 2019, when Wikileaks leaked Vatican documents.
"This and other publications have confirmed the world's suspicion that Mr Assange is still linked to WikiLeaks and therefore involved in interfering in internal affairs of other states."
His accusations against Mr Assange also included blocking security cameras at the embassy, accessing security files and confronting guards.
Mr Moreno said the British government had confirmed in writing that Mr Assange "would not be extradited to a country where he could face torture or the death penalty".
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