Nixon had said last week that he would issue the order; a grand jury decision is expected at any time.

The governor’s state-of-emergency declaration came after federal officials issued a nationwide law-enforcement bulletin warning that the grand jury’s decision “will likely” lead to attacks on police officers or federal agents.

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“The announcement of the grand jury’s decision … will likely be exploited by some individuals to justify threats and attacks against law enforcement and critical infrastructure,” the FBI said in an intelligence bulletin that was first reported by ABC News. Federal officials confirmed the warning — and its wording — to The Post on Monday night.

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One federal official told The Post that it’s not unusual for the FBI to share intelligence with state and local law enforcement agencies. The official said there is a “heightened sense of concern” about Ferguson.

In the days that followed officer Darren Wilson’s fatal shooting of 18-year-old Michael Brown, protesters and police clashed in the streets of the St. Louis suburb, reviving a national conversation about the treatment of African Americans by law enforcement. Heavily armed police faced off with protesters for weeks. Stores were looted, hundreds were arrested, the start of school was delayed and the city was left as a symbol of racial and class divisions in America.

“This is America. People have the right to express views and grievances, but they do not have the right to put fellow citizens and property at risk,” Nixon said last week. “Violence will not be tolerated.”

The grand jury convened Aug. 20 to decide whether to indict Wilson, 28, for the shooting death of Brown on Aug. 9.

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“As part of our ongoing efforts to plan and be prepared for any contingency, it is necessary to have these resources in place in advance of any announcement of the grand jury’s decision,” Nixon said in a statement posted on his Web site Monday afternoon. “All people in the St. Louis region deserve to feel safe in their communities and to make their voices heard without fear of violence or intimidation.

“Public safety demands that we are fully prepared for any contingency, regardless of what the St. Louis County grand jury or the U.S. Department of Justice decides.”

Nixon said in the order that he directed the Missouri State Highway Patrol, St. Louis County Police Department, and St. Louis Metropolitan Police Department to “operate as a Unified Command to protect civil rights” and put the St. Louis County Police Department in charge of security in Ferguson related to protest areas and demonstrations.

You can read the executive order here.

The order came hours after dozens of protesters demanded the indictment of Wilson, marching and chanting through the business district of Clayton, where the county government and prosecutor’s office are located.

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The protest, taking place as wintry temperatures dipped into the the low 20s, marked the second consecutive day of demonstrations as the region and country await a decision from the grand jury.

Protesters have spent weeks planning acts of civil disobedience to take place if Wilson is not indicted — an outcome that is widely expected.

Monday’s action was a march that featured stagecraft. Protesters organized a troupe of “scared white people” who lead the way screaming theatrically “oh no, hide! The protesters are coming!”

Following behind them were dozens who held signs and chanted, often pausing to shut down major intersections during the lunch hour.

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“We’re bringing the protests to them,” said Deray McKesson, one of the organizers. “The point is to put the entire county, and country on notice.”

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Protesters were met with some cheers and honks of encouragement from motorists. Other drivers, however, attempted to force their way through the roadblocks, creating several tense moments as protesters stood in front of vehicles trying to push forward. Clayton police officers were on hand, but largely kept their distance from the protests.

At one point, the marchers were met by a single Wilson supporter, who held a green sign on a nearby street corner that proclaimed: “My family and friends support Officer Darren Wilson and the police.”

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The woman, Patty Canter of Clayton, briefly got in a yelling match with some of the protesters, insisting that “all lives matter” in response to their chants of “black lives matter.”

“When the facts are heard, I don’t think all of these protests will stand,” Canter said.

News reports said the Missouri chapter of the Ku Klux Klan is distributing fliers, warning of “lethal force” against violent protesters. The fliers come from the Traditionalist American Knights Of the Ku Klux Klan (TAKKKK), based in Park Hills, Mo., about 75 miles south of Ferguson. In them, violent protesters are warned that they have “awakened a sleeping giant.”

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Steve King, owner of Metro Shooting Supplies in Bridgeton about 15 minutes west of Ferguson, said the store sold 225-250 guns last weekend. Typical weekend sales are 30-40 guns.

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“One hundred percent of them are buying them because of Ferguson,” King said. “These people are afraid….Our classes are booked until after the 1st of the year.” He said purchases of safes have also increased.

King said sales are up “at every gun shop” in the area.

Larimer and Wax-Thibodeaux reported from Washington. Kimberly Kindy, Sari Horwitz and Hunter Schwartz contributed to this story.