With police officers primed to detain him, Assange must find a way of speaking publicly without setting foot outside, raising the possibility of him being forced to speak from a balcony or lean out of a window.
WikiLeaks was tight-lipped about the logistics of Assange's planned appearance at 1300 GMT, with spokesman Kristinn Hrafnsson telling AFP what little he knew could not be discussed for "security reasons".
But Britain's Foreign Office warned that the steps to the embassy were considered British territory while police said officers would take "appropriate" action" if he strayed from the building.
Assange, 41, took refuge in the embassy on June 19th to evade extradition to Sweden where he is wanted for questioning over alleged rape and sexual molestation.
Supporters of the Australian former hacker, granted asylum by Ecuador on Thursday, believe that once in Sweden he could be extradited to the United
Assange's mother expressed confidence Sunday that her son would make it to Ecuador to continue his whistleblowing work despite a tense diplomatic standoff over his asylum.
Asked whether she thought it was realistic Assange would make it to Ecuador Christine Assange said: "I think it is very realistic."
"He's had billions of people around the world supporting him, the US and their allies are almost alone on this one and the support grows day by day," she told ABC 24 from the Gold Coast.
"It could be that the UK government decides to backtrack from this position of being the US lap dog and stands up for its own sovereignty as well as the sovereignty of Ecuador."
WikiLeaks' publication of a vast cache of confidential government files has enraged the US government and his backers fear he could be tried on espionage charges there and face the death penalty.
WikiLeaks announced on Twitter late Saturday that the renowned Spanish lawyer Baltasar Garzon would speak outside the embassy from 1030 GMT on Sunday.
Garzon, known for pursuing Chile's former dictator Augusto Pinochet, is helping Assange's defence.
Despite Ecuador providing a haven for Assange, British Foreign Secretary William Hague has said Britain has no choice but to seek his extradition.
In line with normal diplomatic practice, embassies are considered the territory of the countries they represent and the host country must seek permission to enter the premises.
Britain has angered Ecuador by suggesting it could invoke the Diplomatic and Consular Premises Act of 1987, which it says allows it to revoke the diplomatic immunity of an embassy on British soil and go in to arrest Assange.
Fewer than 10 police officers and a handful of Assange supporters stood outside the embassy on Saturday.
Vaughan Smith, who invited Assange to stay at his home -- Ellingham Hall in Norfolk in eastern England -- for more than a year while he took his case to the Supreme Court, said he visited the Australian at the embassy three days ago.
"He lives in a small room which can hardly be described as comfortable," he told London's Evening Standard newspaper, adding, however, that he was "happiest behind a computer doing his job" and was coping well.
Ecuador meanwhile received powerful backing from regional allies as they warned Britain of "grave consequences" if it breaches diplomatic security at Ecuador's embassy.
Quito had called on its allies from the Venezuela-led so-called Bolivarian Alliance for the People of Our America (ALBA), whose foreign ministers flew to the Ecuadoran economic capital of Guayaquil and displayed full diplomatic support.
"We warn the government of the United Kingdom that it will face grave consequences around the world if it directly breaches the territorial integrity of the Embassy of the Republic of Ecuador in London," said a statement issued at the end of the ALBA meeting Saturday.
It also rejected Britain's "threats vis-a-vis our territorial integrity and sovereignty".
The regional group, which also includes Bolivia, Cuba, Nicaragua as well as a number of smaller Caribbean nations, expressed its "categorical support for Ecuador's sovereign right" to grant asylum to Assange and urged other world nations to reject what it called "Britain's attempts to impose its will by force."
ALBA nations also called on the United Nations to discuss the issue of inviolability of diplomatic installations around the world.
Media reports on Saturday said Australian diplomats believed Washington was targeting Assange for possible prosecution on charges including espionage and conspiracy relating to his WikiLeaks whistleblowing site.
But one expert said he believed this was unlikely.
"There is a dose of fantasy in all this," said Chris Brown, professor of international relations at the London School of Economics.
"The chances of him being extradited to the US from Sweden are non-existent. If the Americans really want him, they would have asked us (Britain) for him," he told AFP.
In 2010, WikiLeaks published hundreds of thousands of US military documents on the war in Iraq and Afghanistan, as well as diplomatic cables that deeply embarrassed Washington.