A witness and activists said security forces had fired large amounts of tear gas at protest points in Khartoum as some demonstrators began to gather. Protesters barricaded some of the city's main thoroughfares with stones and burning tyres, witnesses said.
The main rallies were expected to start in late morning.
The protests mark the third anniversary of huge demonstrations during the 2019 uprising that overthrew long-time autocratic ruler Omar al-Bashir and led to a power-sharing arrangement between civilian groups and the military.
Last October, military leaders toppled the transitional government in a coup, triggering mass rallies that have called on the military to quit politics and continued for more than eight months.
After the military takeover, there were extended internet blackouts in an apparent effort to hamper the protest movement.
Staff at Sudan's two private sector telecoms companies, speaking on condition of anonymity, said authorities had ordered them to shut down the internet once again on Thursday.
Security forces also closed bridges over the Nile between Khartoum and its twin cities of Omdurman and Bahri, another step taken on big protest days to limit the movement of marchers.
In recent days there have been daily neighbourhood protests in the build-up to Thursday's rallies.
On Wednesday, medics aligned with the protest movement said security forces shot dead a child during protests in Bahri, bringing the number of protesters killed since the coup to 103.
There was no immediate comment from authorities, who have previously said that peaceful protests were allowed and casualties would be investigated.