Al-Ayam newspaper reported on Tuesday that the Advisory Council for Human Rights has called for the release of Hasan al-Turabi, the one-time mentor of president Omar al-Bashir.
The call follows the recent release of 32 political prisoners, of which 13 were from Turabi's Popular National Congress (PNC).
"The official spokesman of the Advisory Council for Human Rights, al-Tayeb Haroun, said the council demanded in its meeting yesterday the release of the nine remaining detainees, one of whom is Turabi," al-Ayam newspaper reported.
It did not identify the other eight detainees, but Turabi aide Mohamed al-Amin al-Khalifa said they were PNC members.
He said most of the members released earlier were student leaders, but also included a former government official and a trade unionist.
Sudan is the scene of Africa's
longest civil war
"The 13 who were released are not guilty of any crime," Khalifa said.
He added the 19 other prisoners released alongside the PNC members belonged to the National Democratic Alliance (NDA), which includes several opposition parties.
Analysts say Bashir's government may have released the political prisoners in a bid to broaden support for negotiations to end a 20-year-old civil war.
Turabi, a key figure in the Islamist government set up by Bashir after a coup in 1989, was arrested in February 2001 for crimes against the state following a power struggle with the president.
He was arrested after signing a controversial deal with the Sudan People's Liberation Army (SPLA), which has been fighting for greater autonomy in the mainly Christian and animist south.
Talks between the SPLA and government have agreed on giving the south a vote on independence and on ending Islamic law in non-Muslim areas.
But they have become bogged down on how to share power and wealth, as well as on the status of the capital, Khartoum.