Less than three hours before the announcement of the grand jury decision, Missouri officials held a news conference to urge calm.

At the conference, which was held at the University of Missouri-St.Louis, about five miles from Ferguson, Gov. Jay Nixon, Mayor Francis G. Slay of St. Louis, Charles Dooley, the St. Louis County executive, and Dan Isom II, the director of the state Department of Public Safety, all said that they did not know what the grand jury had decided. But they tried to reassure residents and protesters that free speech would be protected and order maintained.

“Our shared hope and expectation is that regardless of the decision, people on all sides show tolerance, mutual respect and restraint,” Mr. Nixon said.

The governor talked of meeting with faith leaders earlier on Monday and said that churches in the area would be opened as safe havens. He also emphasized that the police and protest leaders had been working with protest leaders in recent months.

“Law enforcement officials continue to maintain open lines of communication with protest leaders to improve the interaction between police and demonstrators and prevent violence,” he said.

Mr. Dooley highlighted the demonstrations in October, which drew thousands of people but were largely peaceful. “Now is the time to show the world that we can act without being destructive,” he said.

Mr. Slay acknowledged that much long-term work needed to be done to repair race relations in the St Louis area, but also warned that violence would not be tolerated. He pointed to a largely calm demonstration that took place on Sunday night in the Shaw neighborhood of St. Louis, during which protesters were allowed by the police to block streets and which did not result in serious injuries.

“We may allow demonstrators to slow down traffic, but we will not allow them to hurt anyone or damage any property,” he said.

The mayor concluded by talking about the work that still needed to be done.

“The world will be watching us,” he said. “They’re going to watch how we handle our disagreement in the coming days and how we make needed change in the coming months and years.”

— CAMPBELL ROBERTSON