April 6 is a symbolic date for Sudan's civilian opposition, marking the anniversary of uprisings in 1985 and 2019 that ended up ousting two leaders who had seized power in coups.
In anticipation of demonstrations, the authorities declared Thursday a non-working day and witnesses said a large military presence had been visible on Khartoum's streets since Wednesday.
Troops also blocked off the Nile bridges linking Khartoum to its suburbs, Omdurman and North Khartoum, witnesses said.
This year, Sudan is still ruled by a military leader, General Abdel Fattah al-Burhan, who seized power in an October 2021 coup, aborting the transition to civilian rule agreed after the 2019 overthrow of Islamist general Omar al-Bashir, who seized power in a 1989 coup.
A new delay to the signing of a deal to restore the transition, which had been rescheduled for Thursday, prompted the civilian opposition to call for nationwide protests instead.
The Forces of Freedom and Change called on Sudanese to demonstrate for "freedom, peace and justice" and against the return of "the old regime", after several high-ranking officials from the Bashir era found roles in the current administration.
Cracks have emerged within the military over security reforms proposed as part of the deal with the FFC.
The signing ceremony had been pushed back "due to a resumption of talks between soldiers", the FFC said.
Analysts say the sticking point has been the integration into the regular army of the powerful paramilitary Rapid Support Forces (RSF), led by Burhan's deputy Mohamed Hamdan Daglo.
The two men have been at loggerheads over the timetable for the RSF's integration and analysts have pointed to a deepening rift between them.
Created in 2013, the RSF emerged from the Janjaweed militia that Bashir unleashed a decade earlier against non-Arab ethnic groups in the western region of Darfur. The militia has since been accused of war crimes.
"The parties are now working tirelessly to finalize discussions on the remaining points," Burhan said on a speech commemorating the anniversary of the 1985 overthrow of Jaafar Nimeiry, which ushered in a brief period of civilian rule.
"The postponement of the signing ceremony was only done with the aim of establishing a solid framework," he said.
The deal sets out a roadmap for the formation of a civilian-led government and the withdrawal of the military from politics.
It is supposed to build on a preliminary agreement reached in December after near-weekly protests since the 2021 coup that have seen 125 protesters killed by security forces, according to pro-democracy doctors.