FERGUSON, Mo. (AP) — A grand jury declined Monday to indict white police officer Darren Wilson in the death of Michael Brown, the unarmed, black 18-year-old whose fatal shooting sparked weeks of sometimes-violent protests and inflamed deep racial tensions between many African-Americans and police.

Within minutes of the announcement by St. Louis County's top prosecutor, crowds began pouring into Fergusonstreets to protest the decision. Some taunted police, shattered windows and vandalized cars. Several gunshots were also heard. Officers released smoke and pepper spray to disperse the gatherings.

Prosecuting Attorney Bob McCulloch said the jury of nine whites and three blacks met on 25 separate days, hearing more than 70 hours of testimony from about 60 witnesses, including three medical examiners and experts on blood, toxicology and firearms.

He stressed that jurors were "the only people who heard every witness ... and every piece of evidence." He said many witnesses presented conflicting statements that were inconsistent with the physical evidence.

"These grand jurors poured their hearts and soul into this process," he said.

As McCulloch was reading his statement, Michael Brown's mother, Lesley McSpadden, was sitting atop a vehicle listening to a broadcast of the announcement. When she heard the decision, she burst into tears and began screaming before being whisked away by supporters.

The crowd with her erupted in anger, converging on the barricade where police in riot gear were standing. They pushed down the barricade and began pelting police with objects, including a bullhorn. Officers stood their ground.

At least nine votes would have been required to indict Wilson. The grand jury met in secret, a standard practice for such proceedings.

Speaking for nearly 45 minutes, a defensive McCulloch repeatedly cited what he said were inconsistencies and erroneous accounts from witnesses. When asked by a reporter whether any of the accounts amount to perjury, he said, "I think they truly believe that's what they saw, but they didn't."

The prosecutor also was critical of the media, saying "the most significant challenge" for his office was a "24-hour news cycle and an insatiable appetite for something — for anything — to talk about."

Brown's family released a statement saying they were "profoundly disappointed" in the decision but asked that the public "channel your frustration in ways that will make a positive change. We need to work together to fix the system that allowed this to happen."

Thousands of people rallied in other U.S. cities, including Los Angeles and New York to protest the decision, leading marches, waving signs and shouting chants of "Hands Up! Don't Shoot," the slogan that has become a rallying cry in protests over police killings across the country.

President Barack Obama appealed for calm and understanding, pleading with both residents and police to show restraint.

"We are a nation built on the rule of law, so we need to accept that this decision was the grand jury's to make," Obama said. He said it was understandable that some Americans would be "deeply disappointed — even angered," but echoed Brown's parents in calling for any protests to be peaceful.

About 100 people holding signs that read "The People Say Guilty!" blocked an intersection in downtown Oakland, California, after a line of police officers blocked them from getting on a highway on-ramp. Minutes earlier, some of the protesters lay on the ground while others outlined their bodies in chalk.

In San Francisco, a few dozen people gathered in the Mission District chanting "No justice, no peace!"

Several hundred protesters marched through downtown Philadelphia, yelling "No justice, no peace, no racist police!" A similar protest of about 50 people in Pittsburgh was short-lived, with activists saying they plan to regroup Tuesday at the federal courthouse.

In Los Angeles, which was rocked by riots in 1992 after the acquittal of police officers in the videotaped beating of Rodney King, police officers were told to remain on duty until released by their supervisors. While about 100 people gathered in Leimert Park, others held a small news conference demanding police change their policies and release as quickly as the names of those killed the names of the officers who shot them.

Denver Mayor Michael Hancock urged protesters to be peaceful in that city, where a civil jury last month found deputies used excessive force in the death of a homeless street preacher. Clergy gathered at a church to discuss the decision, and about a dozen people rallied in a downtown park with a moment of silence.

The Justice Department is conducting a separate investigation into possible civil rights violations that could result in federal charges, but investigators would need to satisfy a rigorous standard of proof in order to mount a prosecution.

The department also has launched a broad probe into the Ferguson Police Department, looking for patterns of discrimination.

The Aug. 9 shooting inflamed tensions in the predominantly black St. Louis suburb that is patrolled by an overwhelmingly white police force. As Brown's body lay for hours in the center of a residential street, an angry crowd of onlookers gathered. Rioting and looting occurred the following night, and police responded with armored vehicles and tear gas.

Protests continued for weeks — often peacefully, but sometimes turning violent, with demonstrators throwing rocks and Molotov cocktails and police firing smoke canisters, tear gas and rubber bullets. Missouri Gov. Jay Nixon briefly summoned the National Guard.

Outside the Ferguson Police Department on Monday night, St. Louis County police used a bullhorn to order a crowd to disperse, saying it had become an unlawful assembly. Protesters defied the orders and some chanted "murderer." Minutes later, four gunshots were heard down the street.

Hours before the decision was made public, Nixon urged people to remain peaceful as he appeared at a news conference with the state's public safety director and the leaders of St. Louis city and county.

"Our shared hope and expectation is that regardless of the decision, people on all sides show tolerance, mutual respect and restraint," Nixon said.

Some black leaders and Brown's parents questioned McCulloch's ability to be impartial. The prosecutor's father, mother, brother, uncle and cousin all worked for the St. Louis Police Department, and his father was killed while responding to a call involving a black suspect in 1964. McCulloch was 12 at the time, and the killing became a hallmark of his initial campaign for elected prosecutor.

Nixon declined to seek the removal of McCulloch in the Brown case, but he also called for McCulloch to vigorously prosecute Wilson, who had been on the Ferguson force for less than three years. Prior to that job, Wilson was an officer for nearly two years in Jennings, another St. Louis suburb.

McCulloch, a Democrat, has been in office since 1991 and was re-elected to another term earlier this month.

Associated Press writers Alan Scher Zagier in Clayton and Andale Gross in Ferguson contributed to this report. 

Associated Press writer Alan Scher Zagier in Clayton contributed to this report. Follow David A. Lieb at: https://twitter.com/DavidALieb .