FERGUSON, Mo. -- Business owners and residents boarded up windows and cleared away debris Wednesday as Ferguson sought a tentative return to normal after two nights of unrest over a grand jury decision's not to indict a police officer in 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 St. Louis, the city neighbouring Ferguson, screaming "Shame, shame." Police locked down the building and called in more than a hundred extra officers. Three people were arrested.

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

The racially charged case has stoked passions nationwide, triggering debates over the relationship between black communities and law enforcement. Since the grand jury's decision was announced Monday night, protesters in cities throughout the country have rallied behind the refrain "hands up, don't shoot," and drawn attention to other police killings.

In New York City on Wednesday, Brown's parents joined the families of two other black men who were unarmed when they died at the hands of police. The families joined arms with civil rights leader Al Sharpton and prayed for justice at the Harlem headquarters of Sharpton's organization, the National Action Network.

As the tension in Ferguson eased somewhat, Wilson broke his long public silence, insisting on national television that he could not have done anything differently in the Aug. 9 confrontation. The officer testified during the grand jury hearings that he felt threatened and that Brown tried to grab his gun, something the Brown family has said they don't believe.

An influx of guardsmen -- reserve troops that state governors can call up during emergencies -- helped make Tuesday night much calmer, although there still were 58 arrests, and demonstrators in Ferguson set fire to a squad car and broke windows at City Hall.

Many residents hoped that the relative calm of the daylight hours would last through the night and into Thursday's Thanksgiving holiday.

A few hours after nightfall, National Guard troops with rifles were posted at intersections and in several parking lots in the area where stores were looted and burned Monday into Tuesday. But there were no protesters in sight.

Earlier in the day, about a dozen people painted over boarded-up windows on businesses in the St. Louis suburb's historic downtown, where National Guardsmen were stationed every few feet and some looked down from rooftops.

Also Wednesday, authorities said a 20-year-old man whose body was found inside a car in Ferguson after Monday night's riots had been intentionally set on fire.

The death of Deandre Joshua is being investigated as a homicide, but police have not said whether it's connected to the violence that broke out after the grand jury announcement. An autopsy determined that Joshua was shot once in the head.

Associated Press writers Andale Gross, Alan Zagier and Phillip Lucas also contributed to this report.