Mali's interim vice president, Colonel Assimi Goita, said on Tuesday that he had seized power after the transitional president and prime minister failed to consult him about the formation of a new government.
He said elections would be held next year as planned.
President Bah Ndaw and Prime Minister Moctar Ouane were arrested and taken to a military base outside the capital on Monday evening, prompting swift condemnation from international powers, some of which called it an "attempted coup".
The two men were in charge of a transitional government created after a military coup in August that ousted President Ibrahim Boubacar Keita. They were tasked with overseeing a return to democratic elections next year.
The streets of Bamako were calm on Tuesday and two sources told Reuters that Ndaw and Ouane were still being held at the military base in Kati. A spokesman for the U.N. peacekeeping mission in Mali, MINUSMA, said it was seeking access to check on the condition of those detained.
The situation could exacerbate instability in the West African country, where Islamist groups linked to al Qaeda and Islamic State control large areas of the north and centre and stage frequent attacks on the army and civilians.
Political instability and military infighting have complicated efforts by Western powers and neighbouring countries to prop up the impoverished nation.
Goita, who led the August coup, orchestrated the detentions after two fellow coup leaders were dropped from their government posts in a cabinet reshuffle on Monday.
In a statement read by an aide on national television, Goita said elections next year to restore an elected government would go ahead as planned.
"The vice president of the transition saw himself obligated to act to preserve the transitional charter and defend the republic," the statement said.
Mali has been in turmoil since a coup to oust president Amadou Toumani Toure in 2012 coincided with an ethnic Tuareg rebellion in the north, which was then hijacked by jihadists linked to al Qaeda.
French forces drove back the Islamists in 2013 but they have regrouped and expanded their reach to neighbouring Burkina Faso and Niger.
Worried about the fallout of Monday's detentions, the United Nations, European Union, United States and regional countries have all demanded the immediate release of the leaders.
"Sanctions will be adopted against those who stand in the way of the transition," Josep Borrell, the EU's foreign affairs chief, said on Twitter.
A delegation from the Economic Community of West African States (ECOWAS) was expected to visit Mali on Tuesday. ECOWAS played a key role in the formation of the interim government after the August coup.
The problems in Mali are part of a recent democratic backslide in West and Central Africa, where strong military factions have taken control or presidents have extended their rule beyond their allotted mandates.
Last month a military council seized power in Chad after the battlefield death of President Idriss Deby. His son, Mahamat Idriss Deby, now runs the country and has promised a transition to a civilian rule similar to Mali's.
But the opposition and civil society say the military holds too much power and fear Deby will not relinquish control.
"There will be repercussions not just for Mali but for the region," said J. Peter Pham, former U.S. special envoy for the Sahel, now with the Atlantic Council.
He said Deby and others will be watching carefully and if they see a post-coup agreement not stuck to in Mali, there will be little incentive for soldiers who seize power to give some of it up.
A Western official said that the example being set in Mali could complicate negotiations with the military rulers in Chad.