In an announcement on state television on Sunday, Col Mamadi Doumbouya said that the country’s borders had been closed, its constitution had been declared invalid and its government had been dissolved.
Mr Doumbouya addressed the nation from state television headquarters, draped in a Guinean flag with about a half dozen other soldiers flanked at his side.
“We will no longer entrust politics to one man; we will entrust it to the people,” the colonel said in the announcement.
A video of the country’s president, Alpha Conde, emerged on Sunday showing the 83-year-old leader tired and disheveled in military custody – although it is not clear when or where the video was taken.
Mr Doumbouya later told France 24 television that Mr Conde was being held in a “secure spot” and had seen a doctor.
The president, who has been in power for more than a decade, had seen his popularity plummet since he sought a third term in office last year, arguing that term limits did not apply to him.
On Sunday night, the military takeover was condemned by the African Union (AU) bloc, who demanded the immediate release of Mr Conde.
In a statement, AU chairman and Democratic Republic of Congo president Felix Tshisekedi and AU Commission head Moussa Faki Mahamat called on the body's Peace and Security Council to meet urgently to examine the situation and take appropriate measures in response.
The Economic Community of West African States also demanded a return to constitutional order in the country on Sunday and threatened to impose sanctions if the president is not freed.
Mr Doumbouya has claimed that he is acting in the best interests of Guinea, citing a lack of economic progress since the country gained independence from France in 1958.
“If you see the state of our roads, if you see the state of our hospitals, you realise that after 72 years, it's time to wake up,” he said.
However, some observers in the country argue that the tensions between Mr Conde and the army colonel come from a recent proposal to cut some military salaries.
When Mr Conde won the country’s first democratic election since independence in 2010, it was hoped his victory would mark a fresh start for the country, which has been mired by decades of corrupt and authoritarian rule.
His opponents have criticised his presidency for failing to improve the lives of Guineans, many of whom still live in poverty despite the country’s vast mineral riches.
Additional reporting by agencies
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