FERGUSON (REUTERS) - Missouri Governor Jay Nixon urged people in the St Louis area to show respect and restraint following a grand jury's decision on whether to criminally charge a white police officer in the August fatal shooting of an unarmed black teenager.

Prosecutors are set to announce the grand jury's decision at 8pm local time (0200 GMT, 10am Singapore time on Tuesday) on Monday. Authorities have stepped up security in and around Ferguson, Missouri, to guard against the kind of rioting that flared in the weeks after incident.

The death of 18-year-old Michael Brown, a resident of a predominantly black city with a white-dominated power structure, sparked weeks of sometimes violent protests in the St Louis suburb and has become a flashpoint for strained US race relations.

"While none of us knows what that (decision) will be, our shared hope and expectation is that, regardless of the decision, people on all sides show tolerance, mutual respect and restraint," Mr Nixon told reporters.

"This is not the time to turn on each other," St Louis County executive Charlie Dooley said at the same news conference. Mr Dooley said he did not want residents to feel they need to barricade themselves in their homes.

"I do not want people to accidentally shoot or harm someone out of fear," he said.

Mr Nixon has called up the National Guard and local officials have been planning a massive police presence to quell any potential violence, particularly if officer Darren Wilson is not indicted in the Aug 9 shooting.

St Louis County prosecutor Bob McCulloch's office plans to announce the finding in Clayton, Missouri, where the grand jury met, spokesman Ed Magee said.

The grand jury, with nine white and three black members, has been meeting since late August, and has heard evidence including witnesses called by the prosecution as well as a private pathologist hired by the Brown family to review the shooting. Nine jurors need to agree to bring charges.

The victim's father, Mr Michael Brown Sr, told a group of protesters outside the Ferguson Police Department that he had been told the grand jury had reached its decision but said he did not know what it was.

"He's calling for peace and he wants people to be not violent," said Mr Byron Conley, 51, who briefly spoke with Mr Brown."I hope we can do this in a peaceful way. I just don't want no one to look at our little town thinking we're a bunch of wild animals. We're really good people here."

White House spokesman Josh Earnest told reporters at the White House that President Barack Obama has "delivered a pretty forceful message" that any protests following the grand jury's decision should be peaceful.

Michael's parents, ministers and community leaders also have urged sympathisers to remain peaceful, whatever the outcome. In St Louis, officials may allow protesters to occupy public spaces and slow traffic so long as they remain peaceful, Mayor Francis Slay said.

"My message to protesters: We will protect your right to peacefully assemble and speak your mind," Mr Slay told reporters."But turning violent and damaging property will not be tolerated."

Ferguson, a city of 21,000 people, has been on edge for weeks as residents await the grand jury's decision. Shop owners in the city, which faced weeks of sometimes violent protests following Michael's death, have boarded up their windows. The local school district and some neighboring districts cancelled classes on Tuesday due to expected protests.

Protesters have said they plan to demonstrate at the Ferguson Police Department and at the county courthouse in Clayton, about 13km to the south, following the grand jury's decision. Many have said they will take to the streets regardless of whether Mr Wilson is indicted, saying the case illustrates long-simmering tensions between black Americans and police.

Dozens of protesters assembled outside the Ferguson Police Department on Monday evening. One man pounded on a metal bin, using it as a drum to rally the group. Police in Clayton placed large barricades around the courthouse and put locks on mailboxes to prevent them from being opened ahead of the announcement.

Lawyers for Michael's family say the teen was trying to surrender when he was shot, while Mr Wilson's supporters say he feared for his life and opened fire in self-defence. Michael was shot at least six times.

Michael is suspected of having stolen cigars from a nearby convenience store shortly before the incident. He and a friend had been walking down the middle of the street when Mr Wilson approached them.

The grand jury could indict Wilson on charges of manslaughter or murder, or simply conclude that it did not have enough evidence to charge him, said Professor Jens David Ohlin, a professor at Cornell Law School who specialises in criminal law. Its decision will likely focus on what happened in the final seconds before the shooting, he said.

"They would have to say, look, there is no specific one piece of evidence that contradicts the police officer's claim that he was acting in self defence", to bring criminal charges, Prof Ohlin said.

"If there isn't any one piece of evidence from those last few seconds that contradicts him, they may determine that they have no 'true bill,' that there are no charges."