BUSINESS owners and residents have boarded up windows and cleared away debris as Ferguson sought a return to normal after two nights of unrest.

The trouble was sparked by a grand jury decision's not to indict a police officer over the shooting death of an unarmed black 18-year-old.

Protesters continued to hold scattered demonstrations, including a group that rushed into City Hall in nearby St Louis screaming "Shame, shame".

Police locked down the building and called in more than 100 extra officers as three people were arrested.

About 200 demonstrators marched through central St Louis and held a mock trial of Darren Wilson, the white police officer who shot and killed Michael Brown during an August 9 struggle.

The racially charged case has stoked passions across the US, triggering new debates over the relationship between black communities and law enforcement.

As the tension eased in Ferguson, Mr Wilson said he couldn't have done anything differently in the deadly confrontation.