Protesters are due to march in Zimbabwe’s capital Harare to demand the resignation of President Robert Mugabe.
The rally is supported by the military which staged a takeover on Wednesday.
Regional branches of the ruling Zanu-PF party as well as war veterans – who until now were loyal to the president – are also saying Mr Mugabe should quit.
Mr Mugabe, 93, had been under house arrest for days, but on Friday he made his first public appearance at a graduation ceremony in the capital.
The military made its move after a power struggle over his successor.
Mr Mugabe sacked Vice-President Emmerson Mnangagwa last week, apparently to pave the way for his wife Grace Mugabe – who is four decades younger than him – to take over the presidency instead.
The military said it was “engaging” with Mr Mugabe and would advise the public on the outcome of talks “as soon as possible”.
Mr Mugabe has led Zimbabwe since it gained independence from Britain in 1980.
Christopher Mutsvangwa – the leader of the influential war veterans’ association – earlier this week called for a huge turnout at the rally.
“We want to restore our pride and tomorrow is the day… we can finish the job which the army started.”
“There’s no going back about Mugabe. He must leave,” he said.
Meanwhile, at least eight out of 10 regional branches of the Zanu-PF party voted on Friday for Mr Mugabe to resign as president and party secretary.
In an unprecedented broadcast, several regional leaders appeared on state TV saying that he should step down.
They also called for Grace Mugabe to resign from the party, and for Mr Mnangagwa to be reinstated in the party’s central committee.
The party’s members agreed to mobilise support for and attend Saturday’s rally. The party is planning to hold a special central committee session over the weekend to discuss the crisis.
Separately, the Zimbabwe Defence Forces (ZDF) said in a statement on Friday it had been approached by organisers of the rally, which it described as a “solidarity march”.
The statement went on: “ZDF is therefore advising the nation that for as long as the planned march remains orderly, peaceful… and without hate speech and incitement to cause violence, it fully supports the march.”
Liberal groups opposed to the president have also backed the protest.
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