Sudan's army has told the nation to expect an announcement following growing protests calling for President Omar al-Bashir to step down.
The army's message sparked jubilant scenes among Sudanese convinced that a coup was under way, but it remained unclear what was happening.
Thousands marched through Khartoum, some chanting: "It has fallen, we won."
Protests against Mr Bashir, who has governed Sudan since 1989, have been under way for several months.
They were originally sparked by a rise in the cost of living, but grew into a broader anti-government movement.
In the early hours of Thursday, several military vehicles were seen entering the large compound in Khartoum that houses the Ministry of Defence, the army headquarters and Mr Bashir's personal residence, AFP reported.
The compound has been the focus of a sit-in by tens of thousands of protesters who have been urging the military to oust Mr Bashir.
Troops have also raided Mr Bashir's Islamic movement, linked to the ruling party, and deployed at key locations in the capital.
Meanwhile, Sudan's state news agency reported that all the political prisoners in the country were being released.
State TV and radio interrupted their programming, with TV broadcasting a message that the army would be making a statement.
The leading protest group, the Association of Sudanese Professionals, said power had to be handed over to a civilian transitional government made up of people who had no links to the "tyrannical regime".
It urged demonstrators to remain at the sit-in to ensure there was no "partial or false solution"