The governor of Missouri, Jay Dixon has declared a state of emergency on Monday, ahead of a grand jury decision in the case of Michael Brown, the black teenager who was shot dead.
In his executive order, Nixon stated the "possibility of expanded unrest", according to a CNN report. The governor said that people had the right to protest peacefully but that citizens and businesses must be protected from violence and damage to their property.
"The State of Missouri will be prepared to appropriately respond to any reaction to these announcements," Nixon said in the executive order.
Brown, an unarmed black teenager, was shot to death by Darren Wilson, a white police officer on 9 August.
The grand jury is to decide whether to indict Wilson.
"In the days immediately following Michael Brown's death, peaceful protests were marred by senseless acts of violence and destruction," the governor said last week.
"That ugliness was not representative of Missouri, and it cannot be repeated," he added.
Governor Nixon signed the executive order to activate the National Guard to support law enforcement on Monday, saying they would assist state and local police as needed.
The state of emergency will expire in 30 days unless extended by another order.
The St. Louis County prosecutor, Bob McCulloch, has said he expects the grand jury to reach a decision in mid-to-late November.
The shooting of Michael Brown has sparked outrage, riots and protests around Ferguson, and has triggered a national debate about police brutality, race relations and civil rights.
A private autopsy, performed by Dr. Michael M. Baden, showed that Brown was shot at least six times. The report showed that Brown could have survived all the shots except for the last blow, a bullet to the top of his head. Baden has been called to testify before the grand jury, an attorney for the Brown family said.