CLAYTON, Mo./FERGUSON, Mo. (Reuters) - Missouri officials plan to announce a grand jury’s decision on whether to criminally charge a white police officer in the August fatal shooting of an unarmed black teenager in Ferguson at 8 p.m. local time (0200 GMT) on Monday, authorities said.

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 U.S. race relations.

Activists in Ferguson and around the United States have been preparing for the grand jury’s decision for weeks.

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.

Missouri Governor Jay 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.

Nixon plans to make a public plea for peace at 5:30 p.m. local time (2330 GMT) in remarks at the University of St. Louis, a spokesman 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, 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, not looting stores, not throwing things at police officers,” said Byron Conley, 51, who briefly spoke with 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. Brown’s parents, ministers and community leaders also have urged sympathizers to remain peaceful, whatever the outcome.

ON EDGE

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 Brown’s death, have boarded up their windows.

On Monday afternoon, businesses along the Ferguson street that bore the brunt of August’s riots were scrambling to board up the few remaining uncovered windows. The local school district and some neighboring districts canceled classes on Tuesday due to expected protests.

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Protesters have said they plan to demonstrate at the Ferguson Police Department and at the county courthouse in Clayton, about 8 miles (13 km) to the south, following the grand jury’s decision. Many have said they will take to the streets regardless of whether Wilson is indicted, saying the case illustrates long-simmering tensions between black Americans and police.

About 20 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. The school district in nearby Hazelwood canceled all after-school activities on Monday following the news.

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

Brown is suspected of having stolen cigars from a nearby convenience store shortly before the incident. Brown and a friend had been walking down the middle of the street when 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 Jens David Ohlin, a professor at Cornell Law School who specializes in criminal law.

Its decision will likely focus on what happened in the final seconds before the shooting, Ohlin 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 defense,” to bring criminal charges, 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.”