NEW YORK (CBSNewYork) — An estimated 25,000-30,000 protesters took to the streets of New York City Saturday to express outrage over the grand jury decisions not to indict the officers involved in the Eric Garner and Michael Brown cases.

The protest remained mainly peaceful. However, at a news conference late Saturday night, Chief of Department James P. O’Neill said two officers were assaulted on the Brooklyn Bridge.

Photos: New York City Protest Against Police Violence

Authorities say two police saw a man attempting to throw a garbage can onto the roadway and attempted to make an arrest. The officers were then reportedly assaulted, kicked in the face and head, by numerous other protesters.

During that incident, O’Neill said police came in possession of a bag containing what appeared to be newly-purchased hammers as well as a black mask.

(Credit: NYPD)

(Credit: NYPD)

One person has been arrested for disorderly conduct, authorities said.

The officers’ injuries are said to be non life-threatening.

“This is where we have to step in and make arrests. We cannot have our officers assaulted,” O’Neill said.

O’Neill also said there were numerous reports of fireworks being set off on Madison Avenue and the Brooklyn Bridge.

Authorities also say a rear window was smashed in an incident on Madison Avenue.

As 1010 WINS’ Gary Baumgarten reported, the crowd chanted “No justice, no peace,” “Black lives matter,” and “Bring the system down,” as more and more protesters filled Washington Square Park on their march to One Police Plaza.

The march spilled into other boroughs over the course of the day and Chopper 2 was overhead marchers as they walked over the Brooklyn Bridge, blocking traffic in both directions, and near Prospect Park.

As CBS2’s Matt Kozar reported, a wall of blue stood guard against a sea of protesters who marched to police headquarters.

“For too long, the injustice has been swept under the rug and now it’s coming to light,” protester Berchell Marcus said.

Protesters hurled verbal jabs at cops, but there were no physical confrontations at police headquarters, Kozar reported.

One protester told Baumgarten it’s not just a problem with the NYPD, but with society in general.

“There is a systematic murder of people of color in this country and it’s institutionalized racism. The people are here to convince the people in power to legislate against this,” he said.

The combination of cars and marchers made a mess of traffic, with CBS2’s Dave Carlin in the middle of it.

Many drivers who said they were trying to avoid the march unhappily found themselves surrounded.

“The traffic, it’s kind of chaotic to say the least,” one man said.

As Carlin reported, there was no point even trying to move or honk—but rather just put it in park until it passed.

“We’re just kind of sitting here waiting,” East Village resident Victoria Sines said.

Organizers for Millions March NYC are demanding justice for Brown, Garner and Akai Gurley, the unarmed man fatally shot by police in a stairwell in East New York last month.

As WCBS 880’s Jim Smith reported, organizers are also demanding a special prosecutor to handle cases like Brown’s, Garner’s and Gurley’s.

According to its website, the Millions March NYC is calling for the indictment of NYPD Officer Daniel Pantaleo in the Garner case.

Garner, a 43-year-old father of six, died in July after police officers attempted to arrest him for allegedly selling loose, untaxed cigarettes in the Tompkinsville section of Staten Island.

In cellphone video of the incident, Pantaleo is seen placing his arm around Garner’s neck in an apparent chokehold and then taking him to the ground after Garner refuses to be handcuffed.

Garner is heard saying repeatedly, “I can’t breathe!” He died a short time later.

Michael Brown, 18, was fatally shot by Officer Darren Wilson in Ferguson, Missouri on Aug. 9.

The grand jury decisions to not charge the officers involved in both the Brown case and Garner’s case have touched off weeks of protests across the country.

Among other demands, the Millions March NYC said it wants the creation of an independent prosecutors office to handle cases of police misconduct and excessive force.

More than 10,000 also marched during a demonstration calling for an end to police violence in Washington, D.C. Saturday afternoon.

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