Ali al-Haj, secretary general of the Popular Congress Party (PCP), was arrested on Wednesday after being summoned for questioning over former leader Omar al-Bashir's rise to power in a 1989 coup, a senior official from his party said.
The investigation into Haj is a significant step against the Islamist political networks that supported Bashir, who was overthrown in April.
The official, who declined to be named, told Reuters Haj reported to investigators on Wednesday evening after the public prosecution summoned him and was then arrested and transferred to Khartoum's Kobar prison, where Bashir is also being held.
Authorities went to Haj's home on Wednesday to inform him he was being summoned for questioning by the public prosecution, Idris Suleiman, the party's political secretary, told Reuters. There has been no official comment.
"Ali al-Haj is a political leader and not a military man to be charged over the 1989 coup and he was abroad when that coup happened," Suleiman said.
Haj has not been questioned yet, the senior official said.
It is unclear when he will be.
Some of Bashir's aides were arrested after his overthrow.
PCP was founded by the late Islamist leader Hassan al-Turabi, who was one of Sudan's most influential political figures, after a dispute with the former ruling National Congress Party (NCP) led by Bashir. The party later became an ally of the NCP.
The Islamist party has complained of being sidelined in Sudan's transitional political process following Bashir's departure.
The current transitional government was formed in September after a power-sharing deal between anti-Bashir groups and the Transitional Military Council that ruled the country after Bashir's overthrow.