Sudan's interim prime minister and at least five senior government officials have been detained by military forces, according to reports.
The office of Prime Minister Abdalla Hamdok said on Facebook that he and his wife had been detained early on Monday in a "complete coup".
Sudan's information ministry said on its own Facebook page that Mr Hamdok had been taken to an undisclosed location.
In a statement to Reuters, it called it a "coup attempt".
The country's main pro-democracy political group, the Sudanese Professionals Association (SPA), urged people to take to the streets.
"We urge the masses to go out on the streets and occupy them, close all roads with barricades, stage a general labour strike, and not to cooperate with the putschists and use civil disobedience to confront them," the group said.
Thousands in Khartoum and Omdurman responded to the call, with video appearing to show protesters blocking streets and setting tyres on fire as security forces used tear gas.
Chants of "the people are stronger, stronger" and "retreat is not an option!" could be heard.
Officials said those being held also included industry minister Ibrahim al Sheikh, information minister Hamza Baloul, Mohammed al Fiky Suliman, a member of the ruling Sovereign Council, and Faisal Mohammed Saleh, a media adviser to Mr Hamdok.
Ayman Khalid, the governor of the state containing the capital, Khartoum, was also arrested, according to the official Facebook page of his office.
The arrests come after weeks of rising tensions between Sudan's civilian and military leaders, including a failed coup attempt in September.
Significant internet and mobile phone outages have also been reported.
NetBlocks, a group which monitors disruptions across the internet, said it had seen a "significant disruption" to both fixed-line and mobile internet connections across Sudan with multiple providers.
"Metrics corroborate user reports network disruptions appearing consistent with an internet shutdown," the advocacy group said.
"The disruption is likely to limit the free flow of information online and news coverage of incidents on the ground."
A military takeover would be a major setback for Sudan, which has struggled with a move to democracy since protests ended Omar al Bashir's long reign.
Josep Borrell, the EU foreign affairs chief, tweeted about his "utmost concern" over the situation, while US special envoy for the Horn of Africa Jeffrey Feltman said Washington was "deeply alarmed".