Automatic gunfire erupted after about 1,000 people moved toward the center of El Fasher, the capital of North Darfur state, UN officials and aid workers said.
Local residents said protesters were angry after losing money in a Ponzi scheme, a pyramid model where money is paid from one investor to the other and presented as profit.
Two members of the crowd told Reuters that security forces opened fire on protesters. One humanitarian aid source said some protesters were also armed and there were exchanges of fire.
"Between four and 10 people have been killed ... and 30 to 40 people injured ... There was intense fighting. We can not say who killed who," said an aid official, asking not to be named.
Aid workers and UN officials took refuge in their compounds during the confrontation.
"There is a lot of confusion ... We don't know if it is police shooting, or civilians, or the Arab militias in town. They lost a lot of money and are very unhappy," said one aid worker, speaking to Reuters by phone.
The fighting had ended by early afternoon but streets were largely deserted and shops shut, residents of the area said.
"They used guns against the protest ... They shot some people," said Dirar Abdullah Dirar, a member of the League of the Victims of the El Mawasir Market, which helped organize the protest to demand the return of investors' money.
Mawasir is a local term for pipes, slang for a swindle.
Another witness in the crowd told Reuters he saw four bodies in the street. "Everything is quiet now but the problem is not solved. We have still not heard how we are going to get our money back," he said.
El Fasher, and Darfur's other major cities, have become thriving commercial centers during the remote western region's seven-year conflict, boosted by foreign cash brought in by aid workers and peacekeepers and accelerating urbanization.
Local residents told Reuters two men set up a business in El Fasher around 10 months ago, taking in cash and other goods and promising returns of more than 50 percent after a month.
Investors received certificates for their goods and did receive payments in the early days of the scheme, one investor said. But the business closed down before last month's national elections, leaving thousands unpaid, he said.
Police arrested a number of men accused of setting up the investment operation, the secretary general of North Darfur's government, Ali Mohammed Ibrahim, told Reuters last week.
No one was immediately available for comment from North Darfur's police or local government on Sunday.
Sudan's state Suna news agency released a statement from North Darfur governor Osman Kebir, dated from Saturday, promising to step up investigations into the scam.
