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.
Ecuador’s president Lenin Moreno said it withdrew Mr Assange’s asylum after his repeated violations to 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.
“I would like to thank Ecuador for its cooperation & metro police uk for its professionalism. No one is above the law.”
Mr Assange, 47, had refused to leave the embassy, claiming if he did he would be extradited to the United States for questioning over the activities of WikiLeaks.