Residents of Ferguson, Missouri, are bracing themselves for the grand jury verdict on the shooting of an unarmed black teen by a white policeman.

The shooting of 18-year-old Michael Brown by Darren Wilson sparked weeks of sometimes violent protests.

Many businesses have boarded up their windows as they expect more demonstrations to follow the grand jury's decision, particularly if Wilson is not charged.

Among those boarded up was a store selling "I Love Ferguson" merchandise intended to raise money for businesses damaged during rioting.

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Paul Morris boards up windows at the "I Love Ferguson" headquarters in preparation for the grand jury verdictKate Munsch/Reuters
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A man spray-paints the word "Open" after boarding up a business in FergusonAdrees Latif/Reuters
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A woman looks out from a launderette in Ferguson that has been boarded upJim Young/Reuters
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Tracy Reust and Daniel Wentz carry a cat out of the boarded-up Ferguson Animal HospitalJoe Raedle/Getty Images

Some suburban St Louis gun dealers have been doing brisk business, particularly among first-time buyers.

Metro Shooting Supplies reported selling two to three times more weapons than usual in recent weeks – an average of 30 to 50 guns each day.

Police spokesman Brian Schellman said "it would be naive" to say the increase has not been driven by concern over the grand jury decision.

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Protesters hold an upside-down US flag outside the Ferguson Police StationAdrees Latif/Reuters
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Demonstrators protest the shooting death of 18-year-old Michael Brown across the road from the police station in FergusonScott Olson / Getty Images
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Demonstrators stand outside the Ferguson Police Department in Missouri as a grand jury decides whether to indict Darren Wilson, a white police officer who fatally shot Michael Brown, an unarmed black teenagerReuters
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Pattie Cantor displays a sign in support of Officer Darren Wilson during a peaceful protest in Clayton, Missouri. The governor of Missouri declared a state of emergency and activated the National Guard ahead of a grand jury decision in the case of a black teenager shot and killed by a white police officerAFP

There is no specific date for a grand jury decision to be revealed, but it is expected before the end of the month.

The grand jury is considering whether there is enough evidence to charge Wilson with a crime and, if so, what the charge should be. If the jury issues an indictment, a separate jury will be selected to decide whether the person is guilty.

Wilson does not expect to face criminal charges from a Missouri grand jury, a police union official said.

Jeff Roorda, business manager for the St Louis Police Officers' Association, said Wilson has been under a lot of pressure and stress but seems confident in the outcome of the grand jury investigation.

"It's fair to say that neither he nor his defence team expect an indictment," Roorda said.

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Police guard the police station as demonstrators protest the shooting death of 18-year-old Michael Brown in Ferguson, MissouriScott Olson / Getty Images
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Police confront demonstrators outside the police station in FergusonScott Olson / Getty Images
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A St Louis County Police officer stands guard during a protest outside the Ferguson Police DepartmentMichael B Thomas/AFP
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Police arrest a protester who was demanding the indictment of Darren Wilson, the white police officer who shot dead 18-year-old Michael Brown in AugustAdrees Latif/Reuters
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A worker hangs a sign on the wall of a new extension to the police station in Ferguson, Missouri, ahead of the grand jury decision on the shooting death of Michael BrownScott Olson / Getty Images

Authorities have said Wilson shot Brown, who was unarmed, following some sort of physical confrontation that occurred after Wilson told Brown and a friend to stop walking down the centre of a street.

Wilson told authorities that the shooting happened after Brown struggled with him for his gun, according to reports by the St. Louis Post-Dispatch and the New York Times that cited unnamed sources. But some witnesses have said Brown had his arms raised – as if to surrender – when the fatal shot was fired.

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Activists paste part of Damon Davis's #AllHandsOnDeck project on boarded-up businesses in FergusonAdrees Latif/Reuters
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Artist Damon Davis poses in front of his art project called #AllHandsOnDeck, which features pictures of hands in a raised position, pasted on boarded-up shop fronts in FergusonAdrees Latif/Reuters
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Branden Turner cuts Deverain Word's hair at the Prime Time barber shop on West Florissant, where business has dropped nearly 80% since the 9 August shooting death of Michael BrownScott Olson / Getty Images
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Kathleen McGee sings at the St John Missionary Baptist Church in St Louis during a service praying for peace in any reaction to the grand jury decision in the case against Ferguson police officer Darren WilsonJoe Raedle/Getty Images
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Aaron Jeremiah from Houston leaves a memento at a memorial near the location where 18-year-old Michael Brown was shot in FergusonScott Olson / Getty Images