A white rookie police officer will not face charges over the killing of black youngster Tamir Rice, who was shot while playing with what turned out to be a pellet gun, a grand jury in Ohio has decided.

Explaining the decision, Cuyahoga County prosecutor Tim McGinty said it was "indisputable" that Tamir, 12, was drawing the pistol from his waistband when he was gunned down.

Mr McGinty said Tamir was trying to either hand the weapon to police or show them it was not real, but the officer and his partner had no way of knowing that.

"Simply put, given this perfect storm of human error, mistakes and miscommunications by all involved that day, the evidence did not indicate criminal conduct by police," he said.

He said policeman Timothy Loehmann was justified in opening fire, adding: "He had reason to fear for his life."

But Mr McGinty said police radio operators contributed to the tragedy by failing to pass along the "all-important fact" that the caller to the emergency dispatcher said the gunman was probably a juvenile and the gun was probably not real.

Tamir's family condemned the decision but echoed the prosecutor in urging those disappointed to express themselves "peacefully and democratically".

Barricades were set up outside the county court in Cleveland in case of protests and about two dozen people gathered in the rain at the recreation centre where Tamir was shot, some holding signs with photos of the boy and others killed by police in the US.

A grainy surveillance camera video of the November 2014 shooting provoked outrage nationally and together with other killings of black people by police in places such as Ferguson, Missouri, and New York City, helped fuel the Black Lives Matter protest movement.

There was no immediate comment from Mr Loehmann after the decision. A lawyer for his partner, patrolman Frank Garmback, called the shooting a "tragic incident" but said it was clear the officers "acted within the bounds of the law". The grand jury also declined to indict Mr Garmback.

Tamir was shot by Mr Loehmann within two seconds of the officers' police cruiser skidding to a stop near the boy. Mr Loehmann and Mr Garmback were responding to a call to the emergency dispatcher about a "guy" pulling a gun out of his trousers and pointing it at people.

Tamir was carrying a borrowed airsoft gun that looks like an actual firearm but shoots non-lethal plastic pellets, but was missing the orange tip that is supposed to show that it is not a real weapon.

The grand jury had been hearing evidence since mid-October.

Assistant prosecutor Matthew Meyer said it was "extremely difficult" to tell the difference between the pellet gun and the firearm it was modelled on and said Tamir was big for his age - 5ft-7ins and weighing 12st 5lbs - appearing much older than 12.

Mr McGinty also noted that the neighbourhood had a history of violence and a short distance away were memorials to two Cleveland police officers shot dead in the line of duty.

Cleveland police plan to put dashboard cameras in every patrol car and officers who patrol the streets have been equipped with body cameras since September. The city also reached a settlement with the US Department of Justice this year to introduce numerous reforms, including an overhaul of the police department's use-of-force policies.

The settlement was prompted in part by a November 2012 high-speed car chase that ended with the killing of a couple in a 137-shot barrage of police gunfire.

In a statement, Tamir's family said it was "saddened and disappointed by this outcome - but not surprised". It accused the prosecutor of "abusing and manipulating the grand jury process to orchestrate a vote against indictment".

The family accused Mr McGinty of improperly hiring use-of-force experts to tell the grand jury that Mr Loehmann's actions were reasonable and repeated a request for "a real investigation" by the Justice Department.

The family has filed a federal civil rights lawsuit against the two officers and the city and federal prosecutors in Cleveland said a civil rights investigation into the shooting was already under way.

Cleveland mayor Frank Jackson said the city and the police department would conduct an internal review that could result in disciplinary action against the two officers, who have been on restricted duty since the shooting.

Mr McGinty said it was a "tough conversation" with Tamir's mother when she was told there would be no charges. "She was broken up, and it was very hard," the prosecutor said.

Mr Loehmann opened fire from a distance estimated at four and a half to seven feet, discharging two shots, one of which missed.

"With his hands pulling the gun out and his elbow coming up, I knew it was a gun and it was coming out," Mr Loehmann said in a statement he read to the grand jury.

"I saw the weapon in his hands coming out of his waistband and the threat to my partner and myself was real and active."

After the boy's killing, it emerged that Mr Loehmann had left the police force in suburban Independence before he could be sacked because he had a "dismal" handgun performance, broke down in tears at the gun range and was emotionally immature, according to documents.

Steve Loomis, the head of Cleveland's largest police union, said the organisation was pleased with the grand jury's finding but added the decision "is no cause for celebration, and there will be none".

Outside the recreation centre, protesters chanted: "No justice, no peace!"

One, Art Blakey, from Cleveland, said: "There never has been any justice in these police murders. We're supposed to swallow these things whole as if this is business as usual."