Protesters were seen cowering behind cars as shots were fired at the demonstration in Missouri to mark two years since the unarmed black teen was shot dead by police.
A man, named locally as Jacoby Thomas, was also injured in a hit-and-run after a car sped through a group of protesters who were blocking a street.
Witnesses say the vehicle hit the young man so hard that he flew into the air. He was rushed to hospital where is condition is unknown.
'A lady came down and hit a protester - knocked the shoes off his feet,' said Sharon Cowan, who was at the scene. 'Hit him, and he rolled and he bounced.' Heather De Mian, who was at the protests, said she screamed when the man was hit.
The car sped off without stopping after hitting the protester, according to witnesses. Moments later, shots rang out at the demonstrations.
'Then when the bullets flew, I started screaming some more,' she said.
Police responded to reports of gunfire but had found no evidence that anyone had been struck, said Ferguson spokesman Jeff Small, who declined to speculate about why the shots were fired.