"Julian Assange, 47, has today, Thursday 11 April, been arrested by officers from the Metropolitan Police Service (MPS) at the Embassy of Ecuador," police said.
Police said they arrested Assange after being "invited into the embassy by the Ambassador, following the Ecuadorean government's withdrawal of asylum."
Assange took refuge in Ecuador's London embassy in 2012 to avoid being extradited to Sweden, where authorities wanted to question him as part of a sexual assault investigation. That probe was later dropped, but Assange fears he could be extradited to face charges in the United States, where federal prosecutors are investigating WikiLeaks.
Assange was taken into custody at a central London police station and he will be brought before Westminster Magistrates' Court, police said.
Assange's relationship with his hosts collapsed after Ecuador accused him of leaking information about President Lenin Moreno's personal life. Moreno had previously said Assange has violated the terms of his asylum.
Moreno said that he had asked Britain to guarantee that 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, in accordance with its own rules," Moreno said.
British Foreign Secretary Jeremy Hunt thanked Moreno for breaking the impasse, saying on Twitter that Assange "is no hero and no one is above the law."
WikiLeaks said Ecuador had illegally terminated Assange's political asylum in violation of international law.
To some, Assange is a hero for exposing what supporters cast as abuse of power by modern states and for championing free speech. But to others, he is a dangerous rebel who has undermined the security of the United States.