Opposition leaders have complained of mounting pressure since the secession of the oil-producing South Sudan a year ago.
Sudan is facing multiple armed insurgencies in its regions and an economic crisis worsened by the loss of the South’s oil production, previously its main source of state revenues.
Sudanese authorities arrested Ibrahim al-Sanousi, an assistant to the Popular Congress Party’s leader Hassan al-Turabi, in December after he returned from a trip that included Kenya and South Sudan, the party said at the time.
He was released along with Ali Shamar, another party official, on Monday, after responding to questioning from the prosecutor, the state-linked Sudanese Media Centre reported.
Security services had accused the two of maintaining a “direct relationship” with rebel groups, including the Sudanese Revolutionary Front, an alliance of insurgents in Darfur and the southern border states of Blue Nile and South Kordofan, it said.
Kamal Omer, a PCP official, said: “I think the release goes back to the weak information before them and the pressure which the government is living under.”
Veteran Islamist politician Turabi was a close ally of Sudan’s President Omar Hassan al-Bashir, who took power in a 1989 coup, and exercised major influence over Sudanese politics for much of the 1990s.
The two fell out and Turabi has since become one of the government’s most outspoken critics, predicting Sudan would see an Arab Spring style revolt.