Washington: The anger that had been growing in over 100 days of continuous protest spilled into violence in Ferguson, Missouri, last night as the prosecuting attorney, Robert McCulloch, gave a long and at times defensive statement outlining how a grand jury had found no probable cause to indict police officer Darren Wilson for the fatal shooting of unarmed black teen Michael Brown.

Brown's parents had called for people to observe 4½ minutes of silence upon hearing the decision, reflecting the 4½ hours their son had lain on the street after being shot in August. Before Mr McCulloch had even finished speaking, it was clear that some protesters would ignore calls for peace.

As the protests in Ferguson turned violent, authorities in cities across the country monitored other demonstrations. Hundreds gathered in New York's Times Square. Earlier in the evening a rally had been held in Cleveland, Ohio, where demonstrators gathered in support of the family of 12-year-old Tamir Rice, who had been shot dead by a police officer on Saturday as he played with a realistic toy gun in a local park.

In Philadelphia protesters chanted, "No Justice, No Peace, No Racist Police", CBS news reported, while police forces across the country are preparing for possible unrest in the coming days.

Mr McCulloch did not address the public until after 8pm. He told assembled media that the grand jury had heard evidence that contradicted public statements that Brown had been shot dead while trying to surrender, statements that prompted the Ferguson's slogan of "Hands Up, Don't Shoot".

He said that Mr Wilson stopped Brown on the street after recognising him from the description of two youths who had stolen cigarellos from a local convenience store.

During a struggle in the car two shots were fired and Brown was wounded. According to Mr McCulloch, Brown then ran from the car, only to turn and "charge" at the police officer, who fired the fatal shots.

He said much of the physical evidence as well as witness statements contradicted testimony earlier made public that said Brown was trying to surrender.

"Physical evidence does not look away as events unfold," he said.

He said the three-month investigation supported by the FBI was made more difficult by a 24-hour news cycle and social media.

"Within minutes [of the shooting] various accounts of the incident began appearing on social media, accounts filled with speculation and little, if any, solid or accurate information," he said.

"The most significant challenge encountered in this investigation has been the 24-hour news cycle and its insatiable appetite for something, for anything, to talk about, following closely behind with the non-stop rumours on social media."

As police sought to establish calm in Ferguson, US President Barack Obama addressed the nation, saying: "First and foremost, we are a nation built on the rule of law, and we need to accept that this was a decision that was the grand jury's to make."

He reminded viewers that Brown's father had called for his son to be remembered peacefully. Echoing the tone of an interview he had given on Sunday, he acknowledged the anger of African-American communities, but said he believed race relations in America were improving.

"We have made enormous progress in race relations over the course of the last several decades. I've witnessed that in my own life, and to deny that progress, I think, is to deny America's capacity for change," he said.

"But what is also true is that there are still problems, and communities of colour aren't just making these problems up. Separating that from this decision, there are issues in which the law too often feels as if it is being applied in a discriminatory fashion."

Last night there was criticism that authorities waited until after dark to announce the grand jury's decision, inflaming some protesters.

By midnight the store from which Brown is accused of stealing cigarellos had been looted and at least one other building had been lost to a fire.

Police cars had been vandalised and protesters said tear gas had been deployed, though authorities denied its use.