Members of the Sudanese interim government and other civilian leaders were arrested after reports of a military coup.
Prime Minister Abdallah Hamdok is also said to have been placed under house arrest by unknown soldiers.
In the capital, Khartoum, pro-democracy demonstrators took to the streets.
Since the overthrow of long-time ruler Omar al-Bashir two years ago and the establishment of the interim government, military and civilian leaders have been at odds.
Video footage from the North African capital on Monday showed protesters man lighted barricades and enter the area near the military headquarters. There have been some reports of gunfire, but these have not been confirmed.
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No one in the military has commented on this yet, and it remains unclear who was behind the pre-dawn arrests.
A statement by the Ministry of Information on Facebook said the arrests were carried out by “joint armed forces” and that those arrested are being held in “an unidentified location”.
The ministry said soldiers stormed the state broadcaster’s headquarters in Omdurman and arrested employees there.
It also said that Mr. Hamdok was pressured to support a coup but refused to do so, and he urged people to continue peaceful protests to “defend the revolution”.
British special envoy for Sudan and South Sudan, Robert Fairweather, tweeted that the military arrest of civilian leaders was “a betrayal of the revolution, transition and the Sudanese people.”
The US, the UN, the EU and the Arab League have also expressed deep concern.
Witnesses say the internet is down and that army and paramilitary forces are stationed across the city. Khartoum Airport is now closed and international flights are suspended.
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Sudan’s main pro-democracy group has called on its supporters to oppose any military coup.
Tensions in Khartoum have built up rapidly in recent weeks. A hostile takeover is what many in Sudan and beyond have feared it could happen anytime. The signs were all too clear.
A pro-military sit-in right in front of the presidential palace in Khartoum was considered choreographed to lead to a coup. No attempt was made to obscure its purpose. The protesters demanded that the military overthrow “failed” civilian leaders. It was an unusual attempt to legitimize a military takeover under the guise of a popular protest.
Less than a week later, there was a counter-protest. This time, huge crowds came to support the civil government.
With further protests called by pro-democracy groups to “counter a military coup”, Sudan could face another phase of the showdown between the armed forces and the people.
The country has made great strides in normalizing relations with the West and developing much-needed funding flows. The promise of democratic transition has made many Sudanese and the country’s allies hopeful. But all of that could be in danger now.
The military and civil interim authorities have ruled together since the fall of President Bashir after months of street protests in 2019.
A power-sharing agreement between the military and a loose coalition of groups – the Forces for Freedom and Change (FFC) – had been agreed, which brought the Sovereign Council into being.
It would rule the country for another year – with the aim of holding elections and moving to civilian rule.
But the deal has always been divided, with large numbers of rival political groups – and within the military as well.
Tensions increased after a coup attempt attributed to supporters of Mr Bashir was foiled in September.
Sudan has not found a functioning political system since independence in 1956 and has seen numerous coups and coup attempts.