Burkina Faso army chiefs have backed a military officer as interim leader.
In a statement, the army said Lt-Col Isaac Zida was "chosen unanimously to lead the transition period".
This appears to end confusion over who has succeeded President Blaise Compaore, who stood down amid unrest.
But opposition groups have called for a mass meeting on Sunday morning in the capital Ouagadougou, warning the military not to hijack the public revolt against Mr Compaore.
"The victory born from this popular uprising belongs to the people, and the task of managing the transition falls by right to the people. In no case can it be confiscated by the army," they said in a statement.
The BBC's Thomas Fessy says that in a country where the outgoing president has dominated the political scene for nearly three decades, the opposition is weak.
The turnout at the rally will be a measure of its credibility, our correspondent adds.
The army statement was signed by army chief General Honore Traore, who had earlier declared himself head of state.
Mr Compaore, who quit after 27 years as president, fled the country and is now in Yamoussoukro, the capital of neighbouring Ivory Coast.
A statement from the office of President Alassane Ouattara said Ivory Coast took in Mr Compaore, his family and those close to him and was paying "particular attention" to events in Burkina Faso, news agency AP reported.
The BBC's Thomas Fessy, monitoring events from Senegal, says celebrations at Mr Compaore's departure are being replaced by concerns that this popular uprising may have been jeopardised by yet another military coup.
Lt-Col Zida, previously second in command of the presidential guard, was, like Gen Traore, very close to the former president, and his rise raises questions over the army's intentions.
Under the country's constitution, the president of the Senate should take over after the national president resigns, with elections taking place between 60 and 90 days afterwards.
The African Union called for a "civilian-led transition" culminating as soon as possible in "the holding of free, fair and transparent elections".
In a statement, AU chief Nkosazana Dlamini-Zuma urged the military to "refrain from any acts or statements that may lead to further instability".
Protests began earlier this week after Mr Compaore sought to amend the constitution and extend his long hold on the presidency.
On Thursday, protesters set fire to parliament and government buildings in Ouagadougou.