Clashes have broken out between protesters and riot police in the restive town of Ferguson before a grand jury decides whether to indict Darren Wilson, the police officer who shot dead black teenager Michael Brown.

The tensions between the African-American community in the town and authorities are rising despite an appeal from Brown's father for the protesters not to resort to violence.

"My family and I are hurting. Our whole region is hurting," Michael Brown Sr. said in a video.

"I thank you for lifting your voices to interracial profiling and police intimidation. But hurting others or destroying property is not the answer.

"No matter what the grand jury decides, I do not want my son's death to be in vain. I want it to lead to incredible change, positive change, change that makes the St Louis region better for everyone."

The ruling, which hacker group Anonymous have claimed will see Wilson not charged with any crime, has left the St. Louis community waiting for a verdict that would see Wilson charged with the murder of Brown.

"It's fair to say that neither he nor his defense team expect an indictment," Jeff Roorda, St. Louis Police Officers' Association, told the Associated Press.

The Brown family are set to consider civil action if Wilson is not indicted in a criminal court, according to one of their lawyers.

Earlier this week, Governor Jay Nixon declared a state of emergency in Missouri, giving rise to predictions that the controversial grand jury decision about whether or not to charge Wilson will be announced in the coming days.