There was no immediate comment from the soldiers, who hail from the very same military barracks in Kati where an earlier coup originated more than eight years ago.
African Union chairperson Moussa Faki Mahamat condemned the “forced detention” of Malian leaders and called for their immediate release.
He rejected “any attempt at the unconstitutional change of government”.
The developments were also condemned by the United States, the United Nations, the regional bloc known as ECOWAS and former coloniser France, which along with a UN peacekeeping mission has worked since 2013 to stabilise the West African nation.
UN Secretary-General Antonio Guterres sought “the immediate restoration of constitutional order and rule of law,” UN spokesman Stephane Dujarric said.
Tuesday’s political upheaval threatened to further jeopardise security in Mali, where the 2012 coup allowed an Islamic insurgency to take hold amid a power vacuum.
The detention was a dramatic change of fortune for Mr Keita, who seven years earlier emerged from a field of more than two dozen candidates to win Mali’s first democratic election post-coup in a landslide with more than 77% of the vote.
Regional mediators from ECOWAS failed to bridge the impasse between Mr Keita’s government and opposition leaders, creating mounting anxiety about another military-led change of power.
Then on Tuesday, soldiers in the garrison town of Kati took weapons from the armoury at the barracks and detained senior military officers.
Anti-government protesters immediately cheered the soldiers’ actions, some even setting fire to a building that belongs to Mali’s justice minister in the capital.
Hours earlier, Prime Minister Boubou Cisse urged the soldiers to put down their arms.
“There is no problem whose solution cannot be found through dialogue,” he said in a communique.
But armed men began detaining people in the capital of Bamako too, including the country’s finance minister, Abdoulaye Daffe.
Mr Keita, who has tried to meet protesters’ demands through a series of concessions since the demonstrations began, has also enjoyed broad support from France and other Western allies.
“The US is opposed to all unconstitutional changes of government whether in the streets or by security forces,” tweeted J Peter Pham, the State Department’s special envoy for the Sahel region.
It was a troubling repeat of the events leading up to the 2012 coup, which unleashed years of chaos in Mali when the ensuing power vacuum allowed Islamic extremists to seize control of northern towns.
Ultimately a French-led military operation ousted the jihadists, but they merely regrouped and expanded their reach during Mr Keita’s presidency into central Mali.
On March 21, 2012, a similar mutiny erupted at the Kati military camp as rank-and-file soldiers began rioting and then broke into the camp’s armoury. After grabbing weapons, they later headed for the seat of government under the leadership of Captain Amadou Haya Sanogo.
Capt Sanogo was later forced to hand over power to a civilian transitional government, which then organised the election Keita won.
The current president has faced growing criticism of his government’s handling of the Islamic insurgency engulfing the country once praised as a model of democracy in the region.
A wave of particularly deadly attacks in the north last year prompted the government to close its most vulnerable outposts as part of a reorganisation aimed at stemming the losses.
Regional mediators have urged Mr Keita to share power in a unity government, but those overtures were swiftly rejected by opposition leaders who said they would not stop short of Mr Keita’s ouster.
