WELDON SPRING, Mo. — Gov. Jay Nixon said Tuesday that the Missouri National Guard was part of a multiforce contingency plan by law enforcement authorities to avert violence as the region around St. Louis awaited a grand jury’s return in the death of Michael Brown three months ago.

“The Guard will be available when we determine it is necessary to support local law enforcement,” Mr. Nixon announced in a news conference at a state highway patrol headquarters here, standing beside law enforcement leaders from the St. Louis area. “Quite simply, we must and will be fully prepared.”

In August, Mr. Nixon called up the National Guard in the sometimes violent protests that followed the death of Mr. Brown, an unarmed black teenager fatally shot by a white police officer in Ferguson, a suburb of St. Louis. The role of the Guard then was limited to protecting a police command post, but it drew harsh criticism from demonstrators who said it was more a sign of a military-style approach by the authorities. On Tuesday, some demonstrators criticized Mr. Nixon’s newest plan — and the possibility that he would call up the Guard again — as an overreaction that might only heighten potential anger.