Sudan's military seized power in a coup, arresting members of a transitional government that was supposed to guide the country to democracy following the overthrow of long-ruling autocrat Omar al-Bashir in a popular uprising two years ago.
Youths opposed to the takeover barricaded streets and gunfire was heard as demonstrators clashed with the security forces.
Sudan's top general declared a state of emergency and dissolved the transitional government today.
General Abdel Fattah al-Burhan's announcement in a televised address came after armed forces detained government leaders in charge of heading the transition to full civilian rule, following the April 2019 ouster of Mr Bashir.
"To rectify the revolution's course, we have decided to declare a state of emergency nationwide... dissolve the transitional sovereign council, and dissolve the cabinet," said General Burhan, who announced the formation of a new government.
His statement came as clashes erupted in the capital Khartoum, with soldiers firing live rounds at people who took to the streets to protest against the military.
"Civilian rule is the people's choice," demonstrators chanted. "No to military rule".
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The violence, largely centred outside the army headquarters in the capital, came hours after soldiers detained Prime Minister Abdalla Hamdok, ministers in his government and civilian members of Sudan's ruling council, the information ministry said.
They were taken away after "refusing to support the coup", it said on Facebook.
UN Secretary-General Antonio Guterres denounced the military takeover and called for leaders to be freed.
"I condemn the ongoing military coup in Sudan," Mr Guterres said on Twitter. "Prime Minister Hamdok and all other officials must be released immediately."
Meanwhile, the UN Security Council is expected to meet in an emergency closed-door session on Tuesday to address the crisis in Sudan, diplomats told AFP.
The session was requested by Britain, Estonia, France, Ireland, Norway and the United States, the diplomats said.
Internet services were cut across the country around dawn and the main roads and bridges into Khartoum were shut, before soldiers stormed the headquarters of Sudan's state broadcaster in the capital's twin city of Omdurman, the information ministry said.
People took to the streets soon after, setting tyres ablaze and piling rows of bricks across roads in protest against the military move, an AFP correspondent reported.
"Military forces have fired live bullets on protesters rejecting the military coup outside the army headquarters," the ministry added.
Around a dozen people were wounded, said the independent Central Committee of Sudan Doctors.
Under a 2019 power-sharing deal after the ouster of Mr Bashir, Sudan was ruled by a sovereign council of civilian and military representatives tasked with overseeing a transition to a full civilian government.
But in recent weeks the cracks in the leadership had grown wider.
Jonas Horner, from the International Crisis Group think-tank, called it an "existential moment for both sides".
"This kind of intervention... really puts autocracy back on the menu," he warned.
The power grab by the army was condemned by the international community, with the European Union calling for the "fast release" of the civilian leadership, and the African Union and Arab League also expressing concern.
US Special Envoy for the Horn of Africa Jeffrey Feltman said Washington was "deeply alarmed at reports of a military takeover of the transitional government".
Mr Bashir, who ruled Sudan with an iron fist for three decades - and is wanted to face charges of genocide in Darfur by the International Criminal Court - is jailed in Khartoum.
But UN High Commissioner for Human Rights Michelle Bachelet warned Sudan risked stepping backwards towards autocracy.
"It would be disastrous if Sudan goes backwards after finally bringing an end to decades of repressive dictatorship," Ms Bachelet said. "The country needs to move forward to consolidate democracy."
In recent days, rival protests have been held, with sit-ins outside the presidential palace demanding a return to "military rule", and in response, tens of thousands marching to back the full transfer of power to civilians.
The two sides represent opposing factions of the Forces for Freedom and Change (FFC), the civilian umbrella group which spearheaded demonstrations that led to the army's overthrow of Mr Bashir.
Tensions between the two sides have long simmered, but divisions ratcheted up after a failed coup on 21 September this year.
Today, the mainstream FFC appealed for nationwide "civil disobedience", calls also made by Sudan's bankers' association and doctors' union.
The Sudanese Professionals Association, an umbrella group of trade unions which were key in leading the 2019 anti-Bashir protests, denounced what it called a "military coup" and urged demonstrators "to fiercely resist" it.
Protesters were seen marching through the streets of Khartoum carrying the Sudanese flag.
"We will not accept military rule and we are ready to give our lives for the democratic transition in Sudan," said demonstrator Haitham Mohamed.
"We will not leave the streets until the civilian government is back," Sawsan Bashir, another protester, said.
The developments come two days after a Sudanese faction calling for a transfer of power to civilian rule warned of a "creeping coup", at a news conference that was attacked by an unidentified mob.
"The crisis at hand is engineered - and is in the shape of a creeping coup," mainstream FFC leader Yasser Arman told Saturday's news conference in Khartoum.
Mr Hamdok has previously described splits in the transitional government as the "worst and most dangerous crisis" facing the transition.