Sudanese protest against a military coup that overthrew the transition to civilian rule in southern Khartoum. AFP

Protesters were defiant on the streets of Sudan on Tuesday demonstrating against a military coup, as international condemnation of the country's security forces ramped up with the UN Security Council expected to meet later.

"Returning to the past is not an option," chanted the crowds, who remained outside despite soldiers opening fire and reportedly killing at least four people.

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On Monday, soldiers detained Prime Minister Abdalla Hamdok, ministers in his government and civilian members of the ruling council, who have been heading a transition to full civilian rule following the April 2019 overthrow of autocrat Omar Al Bashir.

Sudanese protest against a military coup in the Al Shajara district in southern Khartoum. AFP

UN Secretary-General Antonio Guterres said the detention of the civilian leaders was "unlawful" and condemned "the ongoing military coup d'etat".

UN High Commissioner for Human Rights Michelle Bachelet warned Sudan risked returning to oppression.

"It would be disastrous if Sudan goes backwards after finally bringing an end to decades of repressive dictatorship," she said.

The European Union, African Union and Arab League also expressed concern.

The subsequent declaration of a state of emergency and dissolution of the government provoked an immediate international backlash, with the United States, a key backer of Sudan's transition process, strongly condemning the military's actions and suspending millions of dollars in aid.

The UN demanded Hamdok's "immediate release", while diplomats in New York told AFP the Security Council was expected to meet to discuss the crisis on Tuesday.

Sudanese erect barricades as they protest in the Al Shajara district in southern Khartoum. AFP

Announcing the state of emergency, Sudan's top general Abdel Fattah Al Burhan said the army had taken the actions it had "to rectify the revolution's course".

Internet services were cut across the country and roads into Khartoum were shut, before soldiers stormed the headquarters of the state broadcaster in the capital's twin city of Omdurman.

Telecommunications were interrupted in Sudan, a Reuters witness said on Tuesday, a day after the country's military seized power in a coup and a health ministry official said seven people were killed in clashes between soldiers and street protesters.

There was no official confirmation of the communications interruption. A Reuters witness said internet and phone services were severely limited.