Leftist politician Wagdi Salih, who was freed at a police station in the capital Khartoum, was at the forefront of an anti-corruption committee set up after the ouster of long-time Sudanese ruler Omar al-Bashir in 2019.
The committee, tasked with dismantling Bashir's regime, came under fire from generals who had been sharing power with the civilian Forces for Freedom and Change (FFC) before they staged their takeover, halting a transition that was meant to lead to democratic elections.
Salih was freed on Sunday along with a police officer who had served on the committee. His Forces of Freedom and Change coalition had called his arrest in October "purely political".
Sudan has been without a prime minister since the coup, and its economy, already in crisis, has stagnated after billions of dollars in international financial assistance was suspended.
The military and the FFC announced on Friday that they intended to sign a framework agreement, along with other groups, for a civilian-led transition that would begin after a final deal is signed.
Talks have been facilitated by the United Nations, the United States, Saudi Arabia, and the United Arab Emirates, among others.
(Reporting by Khaled Abdel Aziz; Writing by Nafisa Eltahir and Aidan Lewis; Editing by Gareth Jones and Alexander Smith)