US police in Ferguson, Missouri, have stepped up security ahead of a grand jury decision on the fate of a white police officer, who fatally shot an unarmed African American teenager in August.

St. Louis authorities have set up barricades around the Buzz Westfall Justice Center in Clayton, where a grand jury is considering whether to indict Darren Wilson in the shooting death of 18-year-old Michael Brown.

Local business owners have also taken precautions by boarding up their shops in Ferguson, where Brown was shot multiple times.

St. Louis County prosecutors have not announced the date, time and location, but the some reports say the decision is expected on Monday.

Tensions have been mounting in Ferguson, which has become a pivot of protests against racial discrimination since Wilson shot dead Brown on August 9 in the St. Louis suburb.

Brown’s death triggered weeks of protests and clashes between police and demonstrators in Ferguson. At times, the police used unnecessary and disproportionate force.

Missouri Governor Jay Nixon has declared a 30-day state of emergency and called in 400 National Guard troops to support local police.

Human Rights Watch, a leading international human rights organization based in New York, has warned police departments to allow peaceful assembly and avoid using excessive force in the event of renewed protests.

Rights activists say they would conduct separate rallies at federal courthouses across the US regardless of the grand jury's decision and have called for federal charges to be brought against Wilson if he does not face state charges.