Marches are taking place in the capital Khartoum and across the country. Organizers said security services stormed their offices without warning ahead of the Sunday's demonstrations.
Thousands gathered for major demonstrations against Sudan's ruling generals on Sunday, nearly three months after the military ousted President Omar al-Bashir.
The "million-man" march is seen as a test for protest organizers who said that security forces used live rounds to disperse a sit-in on June 3.
Police fired tear gas at protesters in the capital, Khartoum, as crowds gathered for a rally, according to witnesses cited by the Agence France-Presse news agency.
Security forces raided the offices of one of Sudan's main opposition groups, the Sudanese Professionals Association (SPA), the group said late on Saturday, forcing it to nix a press conference ahead of major protests.
The SPA and the military have clashed repeatedly in recent weeks, disagreeing over the timeline for a transition to democracy after the protest movement ousted former strongman President Omar al-Bashir. Since talks between the two groups broke down earlier this month, a violent crackdown has killed at least 128 people across the country, according to protest organizers.
Demonstrators have repeatedly accused the military of attempting a power grab.
"This represents a violation of liberties that is even worse than the regime of the former president," SPA spokesman Ahmed al-Rabie told Reuters news agency after the raid on its offices.
"This is a bad sign for the atmosphere of mediation between the two parties."