Sudanese Congress Party spokesman Mohamed Arabi said 16 members were freed on Sunday.
Sudanese Congress Party spokesman Mohamed Arabi said 16 members were freed on Sunday, but added that "dozens of other figures including the head of the party and his deputy are still imprisoned."
Another opposition group, the Future Forces for Change, announced that four of its members had also been released.
Groups of protesters have staged rallies in Khartoum and some other cities since the authorities announced a 30 percent hike in petrol and diesel prices in November that has led to a sharp rise in the cost of other goods, including medicines.
A previous round of fuel subsidy cuts in 2013 sparked serious unrest that was only suppressed by a deadly crackdown that drew international condemnation.
The government is determined to avoid any repetition of that kind of unrest and has rounded up several opposition leaders to prevent widespread protests.
On December 12, President Omar al-Bashir vowed to crush any new anti-regime protests as the authorities did three years ago.
Rights groups say about 200 lives were lost in clashes with security forces in the 2013 crackdown on street protests, while the government puts the death toll at less than 100.