A photographer runs by a burning building during a demonstration on Nov. 25, 2014, in Ferguson, Missouri, following the grand jury decision not to indict Ferguson police officer Darren Wilson for the fatal shooting of unarmed black teen Michael Brown, on Aug. 9. His death sparked months of sometimes violent protests in the St. Louis suburb. Justin Sullivan/Getty Images

St. Louis Mayor Francis Slay and Police Chief Sam Dotson held a news conference Tuesday morning to condemn the violence that gripped their city following the announcement around 7:15 local time of a grand jury’s decision to not indict police officer Darren Wilson for the killing of Michael Brown. The officials warned that the city’s police department would begin intervening at the first sign of criminal activity, taking a preemptive strategy in dealing with the unrest sweeping the area.

Overnight, police made a total of 21 arrests, many for burglary and trespassing, said Slay, who condemned the violence as “unacceptable and wrong” and a “black eye on the community.” Though the mayor conceded that the protests were largely peaceful, he said the violence that emerged from them "would not be tolerated" and “set back the cause of social justice."

Dotson praised the St. Louis police department for its overnight efforts, saying that the department worked well with the organized groups protesting in the streets and handled itself with “restraint.” However, he expressed his outrage at the criminal elements that were “taking advantage” of the situation. Dotson described one of the burglaries, an incident at a Radioshack on Hampton Avenue, as a case of looting. Of the 21 arrests made last night, 15 were felony violations and six city violations. The mayor also took care to point out that only nine people arrested last night were from Ferguson, the rest from other cities.

Both officials emphasized the importance of a robust police presence in heading off criminal activity and suggested that this presence would become far more visible in St. Louis in the days and nights ahead. However, Dotson pointed out that the area of unrest spanned only three blocks of St. Louis, with the epicenter on Grand Avenue. The police chief said that despite the heightened state of alert from his department, businesses would be open as usual. “Get ready for Thanksgiving, that’s the message,” he said.