A woman holds a sign during a rally at Union Square in New York November 24, 2014. [Photo/Agencies]

A Missouri grand jury voted not to charge a white police officer for the fatal August shooting of an unarmed black teenager, an incident that set off weeks of sometimes violent protests around the St. Louis area, a county prosecutor said on Monday.

The grand jury found there was no probable cause to charge Ferguson, Missouri, police officer Darren Wilson with any crime for the shooting of 18-year-old Michael Brown, an incident that highlighted longstanding racial tensions in the predominantly black city, which has a white-dominated power structure.

"They determined that no probable cause exists to file any charge against officer Wilson," St. Louis County Prosecutor Bob McCulloch told reporters in Clayton, Missouri, a suburb of St. Louis where the grand jury met.

Brown's family said they were "profoundly disappointed" by the grand jury's finding.

"While we understand that many others share our pain, we ask that you channel your frustration in ways that will make a positive change," the family said in a statement released by its attorneys.

A crowd of several hundred people gathered outside the Ferguson Police Department ahead of the news, and many began to scream angrily as the news of the grand jury's decision was read.

"Murderers, you're nothing but murderers," one woman shouted through a megaphone at officers clad in riot gear. "Stinking murderers."

Authorities had stepped up security in and around Ferguson ahead of the decision, and Missouri Governor Jay Nixon called up the National Guard to guard against the kind of rioting that flared in the weeks after the Aug. 9 shooting.

Some activists described the decision to preemptively activate the guard as unnecessarily heavy-handed, particularly following complaints that police inflamed crowds in August by responding in a heavily militarized posture, using tear gas and rubber bullets.

Ahead of the decision, officials called on area residents to remain calm following the grand jury's decision.

The grand jury, with nine white and three black members, began meeting in late August and heard evidence that included witnesses called by the prosecution as well as a private pathologist hired by the Brown family to review the shooting. Nine jurors needed to agree to bring charges.

McCulloch declined to say if the jury's decision was unanimous, noting that grand jury proceedings are completely secret and that only the jury members themselves know the details of the proceedings.

CONFLICTING ACCOUNTS

McCulloch described a tangled mass of conflicting testimony from 60 witnesses about what happened during the incident that led to Brown's death, but said much of it did not square with the physical evidence.

"Many of the same witnesses acknowledged that they did not see the shooting," McCulloch said.

The grand jury met on 25 days and heard more than 70 hours of testimony, McCulloch said.

"The grand jury worked tirelessly to examine and reexamine the testimony of all the witnesses and all the physical evidence."

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.

Witnesses disagreed on whether Brown's hands were up at the time he was shot, McCulloch said, adding that Wilson shot at Brown 12 times, with his final shot hitting Brown in the top of his head.

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. Police said in August that Wilson was not aware of the robbery at the time.

Across the United States, in cities including New York, Chicago and Greensboro, North Carolina, activists were planning to hold demonstrations in solidarity with the Ferguson protesters.

A crowd of about three dozen protesters gathered outside the Clayton courthouse where the grand jury had met, and many stood in stunned silence following the news.

"That's just how the justice system works. The rich are up there and the poor are down here," said Antonio Burns, 25, who lives in Clayton. "They think they can get away with it."