A protester in Berkeley, California. Photo: Reuters

New York: New York state's top prosecutor on Monday sought the power to probe all police killings of unarmed civilians in his state, following sometimes violent protests over two grand juries' moves to clear officers in the deaths of unarmed black men.

The morning after angry crowds hurled objects at police who responded with tear gas during protests in northern California, New York Attorney-General Eric Schneiderman said action was needed to address a "crisis of confidence" in the criminal justice system.

Cities across the United States have seen large protests in recent nights following a grand jury's decision not to charge an officer in the July killing of Eric Garner. An unarmed black father of six, Garner died after police put him in a chokehold.

Officers of the Berkeley Police Department at Monday night's rioting. Photo: Reuters

The decision in the Garner case came a little more than a week after a Missouri grand jury cleared an officer in the August fatal shooting of an unarmed black teenager, Michael Brown.

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"The horrible events surrounding the death of Eric Garner have revealed a deep crisis of confidence in some of the fundamental elements of our criminal justice system," Mr Schneiderman said in a statement. "Nothing could be more critical for both the public and the police officers who work tirelessly to keep our communities safe than acting immediately to restore trust."

Mr Schneiderman said he was seeking a temporary executive order from state Governor Andrew Cuomo shifting authority to investigate police killings of unarmed civilians to the state from local prosecutors, who work closely with local police, until legislators could pass a more permanent measure.

Samaria Rice, the mother of slain 12-year-old Tamir Rice, at a news conference on Monday with her lawyer Benjamin Crump (left). Photo: AP

US Attorney-General Eric Holder, whose office is investigating the Garner case, on Monday plans to unveil a set of changes to federal law enforcement guidelines intended to set an example for local police, according to a Justice Department official.

Cold, wet weather in New York brought smaller crowds of protesters over the weekend, though organisers have vowed fresh actions on Monday as Britain's Prince William and his wife Kate visit the city.

In Cleveland, where the US Justice Department last week said police routinely use excessive force, Samaria Rice, the mother of a 12-year-old African-American boy shot dead by police in November, spoke to media for the first time since her son's death.

A demonstrator in Berkeley, where protests against police shootings turned violent on Monday night. Photo: Reuters

Mrs Rice's son, Tamir, was shot near a recreation centre while carrying a pellet gun that was a replica of a real gun. The boy's family has filed a federal civil rights lawsuit against the city and the two officers involved, one who shot Tamir Rice on November 22 and one who was driving the police car.

"I'm actually looking for conviction," Mrs Rice told reporters. "Tamir was a bright child, he had a promising future and he was very talented in all sports, soccer, basketball, football. He was my baby."

Mrs Rice told reporters that she ran to the scene when two children told her that her son had been shot. Two of her other children also ran to the playground.

Protest: Basketball star Kyrie Irving wears a shirt bearing Eric Garner's last words before an NBA game on Monday night. Photo: AP

Once there, police restrained the elder son and daughter, Mrs Rice said. Her daughter said she had been tackled by police, then handcuffed and put in a police car, the mother said.

Mrs Rice said she was put in the passenger seat of the ambulance instead of being allowed to travel in the back with her son. She also said she didn't see any police officer try to treat Tamir or administer first-aid measures.

As for the toy gun, Mrs Rice said it must have come from a friend. "I don't allow that type of toy in my house. Period. He got it from a friend."

The Rice family has retained Benjamin Crump to represent them. The lawyer also represents the family of Michael Brown, 18, fatally shot by a white police officer in Ferguson, Missouri, on August 9. Mr Crump also represented the family of Trayvon Martin, shot to death on February 26, 2012, in Florida by George Zimmerman, who was acquitted of all charges.

Berkeley riots

Police in Berkeley, California, on Monday said they made five arrests during Sunday night protests, when a crowd of more than 500 people hurled objects at police and a number of stores were looted. One protester who tried to prevent the looting was assaulted, police said.

The two nights of looting and rock-throwing on the West Coast contrasted with mostly peaceful demonstrations elsewhere around the United States following the decision not to indict New York Police Department officer Daniel Pantaleo for the July death of Garner, 43.

While no criminal charges have been brought, and Mr Schneiderman's request would not apply to the Garner case or any other case preceding the executive order he has sought, the NYPD is conducting an internal investigation into the case. The probe into whether Mr Pantaleo broke any department rules could take four months, officials have said. The Justice Department is also investigating the case.

Smaller protests were held in Seattle, Chicago and Miami on Sunday.

The outcry over the recent killings spread to sports stadiums as well. Detroit Lions gridiron star Reggie Bush and Cleveland Cavaliers basketballer Kyrie Irving were among several athletes donning pre-game practice jerseys reading "I can't breathe", Garner's dying words.

Reuters, Los Angeles Times