The head of the National Intelligence and Security Service's media office denied knowledge of the arrests.
Sudan has been rocked by more than a week of anti-government protests sparked by rising prices, shortages of basic commodities and a cash crisis. At least 19 people have died during the protests, including two military personnel, according to official figures.
The statement by a committee of professional organizations involved in the protests said authorities had raided a meeting of opposition leaders in Khartoum. They detained a total of nine people, including Siddiq Youssef, a senior leader of Sudan's Communist Party, as well as leaders from the pan-Arab Ba'ath and Nasserist parties, the statement said.