New York City Mayor Bill de Blasio said Thursday that "we won't tolerate disorder" from protests over policing issues and NYPD Commissioner William Bratton affirmed that the department is taking a "more assertive" stance to prevent a repeat of last December's traffic-paralyzing demonstrations.

Their remarks came a day after the NYPD arrested 143 people in Manhattan who fanned out from Union Square to protest the death in police custody of a man in Baltimore, Freddie Gray.

"We are going to be more assertive in dealing with efforts to close down tunnels, close down bridges and we will be much faster to make arrests if in fact they attempt to move in those directions," Bratton said at an event at the Police Academy in College Point, Queens.

EditorialEditorial: Opposing nonviolent protest is un-AmericanSee alsoComplete coverage

He said police will respect the right to lawful protest, "but when you start having a very significant adverse impact on everybody else's lives, I have to be concerned about that also."

NYPD tactics during Wednesday's demonstrations were assailed by activists and sympathetic elected officials who complained outside police headquarters that officers had shoved, manhandled and arrested even demonstrators who were doing nothing illegal.

"What we saw last night was an unwarranted and exaggerated response, a violent response from the police to a nonviolent protest," said state Sen. Gustavo Rivera (D-Bronx).

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De Blasio, in a City Hall news conference, defended the police response.

"We won't tolerate illegality," de Blasio told reporters. "We won't tolerate disorder."

Bratton said two protesters assaulted officers, one of whom was injured.

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Last December, after a grand jury declined to bring charges against a police officer in the death by apparent chokehold of a Staten Island man, Eric Garner, the city was beset by weeks of street protests that blocked major thoroughfares and river crossings.

Police gave the demonstrators wide leeway, and officers were injured in several clashes, including one on the Brooklyn Bridge. That, and de Blasio's expressions of sympathy with those upset over the Garner case, drew the ire of police unions.

Now, the de Blasio administration is taking a more aggressive stance.

"When police give you an instruction, you follow the instruction. It's not debatable," de Blasio said. He added: "When we think there is the prospect of something unruly or violent, it's necessary to intervene."

De Blasio got testy with reporters asking about the city's response to the protests.

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The mayor, whose campaign in 2013 sided with critics of police stopping and frisking mostly black and Hispanic men, acknowledged that "some people who are complaining today are my colleagues, my friends and have been my supporters."

Asked by a reporter whether he had authorized a harder line than in December, he declined to answer, adding: "If you guys want to sensationalize, if you think that's your contribution to society, feel free."

He said the NYPD had kept the "same strategy" but is making "tactical adjustments."

De Blasio received praise from an unlikely ally: Republican former Mayor Rudy Giuliani, who had warned during the mayoral campaign that New York City would descend into chaos if Democrat de Blasio were elected.

"That's the way to do it," Giuliani said, according to The New York Observer. "What they did . . . is the textbook case in how you handle a demonstration so you don't have a riot."

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With Anthony M. DeStefano

and Alison Fox