Sudan's military has dissolved civilian rule, arrested political leaders and declared a state of emergency.
Gen Abdel Fattah Burhan, who had been heading a joint council with civilian leaders, blamed political infighting.
Protesters have taken to the streets of the capital, Khartoum, and there are reports of gunfire.
Military and civilian leaders have been at odds since long-time ruler Omar al-Bashir was overthrown two years ago and a transitional government set up.
Although the country remains in a deep economic crisis, there had been more international support - and a military takeover will put that hugely at risk.
Video footage from the north African nation's capital on Monday showed protesters manning lit barricades and entering the area near the military's headquarters.
One demonstrator, Haitham Mohamed, told Agence France-Presse (AFP): "We are ready to give our lives for the democratic transition in Sudan."
Prime Minister AbdallahHamdok is among those reported to have been put under house arrest, along with members of his cabinet and other civilian leaders.
A statement from the information ministry on Facebook said those arrested were being held in "an unidentified location".
It also said MrHamdok was being pressed to support a coup but was refusing to do so and he urged people to continue with peaceful protests to "defend the revolution".
Gen Burhan has been heading the Sovereign Council, part of a power-sharing arrangement between military and civilian leaders.
In a televised address, he said infighting between politicians, ambition and incitement to violence had forced him to act to protect the safety of the nation and to "rectify the revolution's course".
He said Sudan was still committed to "international accords" and the transition to civilian rule, with elections planned for July 2023.