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.
Source: Sky News