The African Union on called for a "civilian-led transition" in Burkina Faso after a violent uprising toppled President Blaise Compaore.
AU chief Nkosazana Dlamini-Zuma urged "political stakeholders and civil society in Burkina Faso to work together to agree on a transition that would culminate, as soon as possible, in the holding of free, fair and transparent elections".
An officer in the presidential guard has seized power and promised to lead the west African country to elections.
Lieutenant Colonel Issaac Zida said he was taking control after the resignation of Mr Compaore, who has left for Ivory Coast.
However, opposition parties and civil society groups have called for a mass rally in the capital Ouagadougou tomorrow.
They said they wanted a democratic solution to the crisis and warned against a military takeover.
Army chief of staff General Honore Traore, a Compaore loyalist, quickly announced yesterday he would take over the presidency on a transitional basis.
However, this was rejected by demonstrators and a number of junior officers.
Mr Compaore stepped down after two days of mass protests against his attempts to change the constitution to extend his 27-year rule.
At least three people were killed after protesters stormed the parliament building and set it on fire.
After reports of heavy gunfire near the presidential palace early this morning, Lt Col Zida, the operational commander of the elite presidential guard, announced on radio that he was taking power.
"I assume from today the responsibilities of head of this transition and head of state," he said, dressed in military fatigues, in the studio of BF1 television.
"I salute the memory of the martyrs of this uprising and bow to the sacrifices made by our people."
The unfolding crisis is being closely watched by the United States and former colonial power France, which were close military allies of Mr Compaore.
Under his rule, Burkina Faso became a key ally in Western operations against al-Qaeda-linked groups in west Africa.
The events will also be carefully followed by other governments in west and central Africa, where long-serving leaders are reaching the end of their constitutional terms in several countries, including Benin, Congo and the Democratic Republic of Congo.
Lt Col Zida said the army had stepped in to avoid anarchy and ensure a swift democratic transition.
He said a roadmap to elections would be drafted by a body drawn from different elements of society, including political parties and civil society.
He appealed to the African Union and west African regional union ECOWAS to show its support for the transition.
"This is not a coup d'etat but a popular uprising," he said.
"The people have hopes and expectations, and we believe we have understood them."
