Julian Assange is due to address his supporters from the Ecuadorean embassy in London, marking six months since he was granted asylum inside the building.
It will be the second time the WikiLeaks founder, who took refuge at the embassy in June, has delivered a message from the balcony.
He faces extradition to Sweden over sexual assault claims, which he denies.
Crowds are expected to gather outside the embassy in the Knightsbridge area to watch the speech.
Australian Mr Assange, 41 - whose Wikileaks website published a mass of leaked diplomatic cables embarrassing a number of countries - fears his extradition could lead to him being sent to the US and punished for releasing top secret documents.
Mr Assange delivered a message from a balcony in August, calling for an end to the diplomatic impasse that began when Ecuador's government granted him political asylum.
Sweden wants to question him over allegations that he sexually assaulted two female ex-Wikileaks supporters while he was in Stockholm to give a lecture in 2010.
Mr Assange entered the embassy after the UK's Supreme Court dismissed his bid to reopen his appeal against extradition. It had given him a two-week grace period before extradition proceedings could start.
Mr Assange has been warned he will be arrested when he leaves the embassy for breaking the terms of his bail conditions, and officers from the Metropolitan Police continue to mount a round-the-clock guard on the building.
It is established international protocol that local police and security forces are not permitted to enter an embassy, unless they have the express permission of the ambassador.
In November, Mr Assange told journalists that a move by credit card companies to block the processing of donations to Wikileaks had cost the organisation more than £30m and had resulted in a 40% pay cut for staff.
He said the whistle-blowing website had lost 95% of its revenues and claimed right-wing US politicians were behind the block.