The Sudanese Communist Party said its leader Mohamed Mukhtar Al-Khatib had been grabbed following a visit to Juba where he met with leading Sudanese rebel leaders.
The arrest sent demonstrators out into the streets of Khartoum, marching under the harsh sun as security forces, including the U.S.-sanctioned Central Reserve Police, were deployed at key points along the protest route.
The Communist Party, which has been the most hardline against the coup and any future deal, was pursuing a unified front against the military coup that ousted president Omar al-Bashir in 2019, it said.
Sudan's economy has spiraled as its government has gone without a prime minister since January. Businesses are stagnating while citizens face steep increases in the prices of food, electricity and fuel.
"The military has failed economically, politically and psychologically - in every way," said a 30-year-old protester and unemployed engineer who declined to give his name for fear of retribution. "They are just being stubborn now, but we are more stubborn."