Missouri Governor Jay Nixon has urged tolerance, respect and restraint after a grand jury decision on the fatal shooting of a black teenager by a white police officer in August.
He was speaking after it was confirmed that the grand jury had reached a decision on whether or not to indict Officer Darren Wilson over the shooting of Michael Brown in Ferguson.
Prosecutors said the decision will be announced at 8pm local time (2am Irish time).
There is no indication yet as to whether Officer Wilson will face charges for the shooting of the 18-year-old on 9 August.
Mr Nixon said the National Guard would be deployed in support roles, with the focus on protecting property, people and speech.
The governor had declared a state of emergency in anticipation of the ruling.
The killing sparked major protests in the St Louis suburb and activist groups have pledged new street protests if Officer Wilson is not indicted for the fatal shooting.
The state has been planning a massive police presence to quell any possible violence.
Awaiting the grand jury's decision, shop owners have boarded up store fronts and students in one school district began an extended Thanksgiving break today.
Lawyers for Mr Brown's family said the teenager was trying to surrender when he was shot.
Officer Wilson's supporters said he feared for his life and opened fire in self-defence.
Mr Brown's parents, ministers and community leaders have urged sympathisers to remain peaceful, whatever the outcome.
