GWEN IFILL: Racial tensions and police use of force dominated the nation's agenda today, after the latest high-profile case involving a white officer and a black victim. This time, it happened in New York. But new calls for calm and new cries of anger echoed across city after city.

Around the country, protests built as the day wore on. In Pittsburgh, people laid down in the street, blocking traffic. And, in New York City, with more rallies starting this evening, Mayor Bill de Blasio again urged calm.

BILL DE BLASIO, (D) Mayor of New York: As we said yesterday, we will not tolerate violence or disorder. But we think, by showing respect for the democratic process, it's one of the right ways of setting a tone that keeps the protests peaceful.

GWEN IFILL: Eighty-three protesters were arrested last night in New York, but, as in many cities and on many college campuses, the demonstrations were largely peaceful. They began after a grand jury decided against indicting police officer Daniel Pantaleo in the killing of Eric Garner last July.

Viral video showed police stopping Garner on Staten Island for suspicion of selling untaxed cigarettes.

ERIC GARNER: I did nothing. We were sitting here the whole time minding our business.

GWEN IFILL: When Garner balked at being arrested, Pantaleo wrapped his arm around Garner's neck, wrestling and holding him down. Garner, who had asthma, pleaded that he was suffocating.

ERIC GARNER: I can't breathe. I can't breathe.

GWEN IFILL: Garner died later at a hospital.

A medical examiner ruled it a homicide by a banned chokehold. But, today, the head of the city's police union called Pantaleo's action a textbook maneuver.

PATRICK LYNCH, President, Patrolmen's Benevolent Association: This was a police officer who was sent to that location to do a difficult job, had to bring a person to the ground that said, I'm not going, and was resisting arrest.

While bringing that person to the ground, yes, they said, "We can't breathe." But the police officers and the EMS did what they're supposed to do at that time. If you're speaking, you can breathe.

GWEN IFILL: Mayor de Blasio disagreed. He said today he's ordering a retraining program for the city's police. And civil rights leaders, including the Reverend Al Sharpton, focused on the legal system's handling of the cases in New York, and in Ferguson, Missouri.

REV. AL SHARPTON: We need to centralize and make clear that we want the Justice Department and the federal government to deal with the fact that grand jury systems on a state level are broken and seem to lack the capacity to deal with police.

GWEN IFILL: Attorney General Eric Holder has already ordered a civil rights investigation in the Garner case. And today in Cleveland, he announced findings that that city's police use excessive force far too often.

ERIC HOLDER, Attorney General: I think we certainly see patterns that have emerged through the investigations that we have done, where you see inadequate training, where you see resource deficiencies, where you see cultural — cultural problems that exist within police departments.

GWEN IFILL: That review began even before an officer shot and killed a 12-year-old boy last month who turned out to be carrying a toy gun.

A new agreement calls for a court-appointed monitor to oversee reforms in the Cleveland police force. Civil rights figures are also making plans for a march and summit in Washington later this month.