The group's editor-in-chief, Kristinn Hrafnsson, told a media conference in London on Wednesday that Assange's meetings with lawyers and a doctor had been secretly filmed by the Ecuadorian authorities.
The whistleblower group released letters sent to Assange by embassy officials in which he was asked not to create any interference that would prevent eavesdropping on his meetings - such as playing a radio loudly during meetings or turning a bright light in the direction of surveillance cameras in the room where the meeting was taking place.
Hrafnsson said photos, documents and audio recordings about Assange's activities in the embassy had been sent to WikiLeaks by "Spanish individuals".
ABC News reported that Ecuador Vice-President Otto Sonnenholzner had said Assange would have to abide by the rules laid down by Quito if he wanted to stay in the embassy.
He said that certain rules had been laid down for Assange in order to reduce the costs of providing him asylum. Foreign Minister José Valencia has presented a report about Assange to President Lenín Moreno on Wednesday, and would inform congressional leaders about it a day later.
Assange has been taking refuge in Ecuador's London embassy since June 2012. He faces arrest by the British police if he leaves the embassy for jumping bail.
But WikiLeaks says the thing Assange fears is being extradited to the US to face charges over leaking of classified material, including footage of indiscriminate killing of civilians by US troops who were occupying Iraq.