NEW YORK (REUTERS) - Protesters who have rallied for weeks over excessive use of police force rejected New York City mayor Bill de Blasio's plea to suspend demonstrations after the killing of two officers, vowing to march in the centre of Manhattan on Tuesday evening.

De Blasio and other politicians have called for a cooling of tensions after the officers were ambushed Saturday while sitting in their patrol car, shocking a city that had seen largely peaceful demonstrations over decisions by grand juries in New York and Missouri not to indict white police officers in the killings of unarmed black men.

Since Saturday, some activists have woven in protests against the killings of officers Rafael Ramos, 40, and Wenjian Liu, 32, into their rallies and vigils.

Answer Coalition, organisers of a march on 5th Avenue in midtown Manhattan said a "peaceful protest against police violence" would continue as planned.

"The mayor's call for a suspension of democracy and the exercise of free speech rights in the face of ongoing injustice is outrageous."

The point-blank shootings of Ramos and Liu electrified tensions between City Hall, the police department and the reform-minded protesters who voted for de Blasio, a liberal Democrat, in large numbers last year to run the largest US city of eight million people.

Similar protests, some of them violent, have taken place across the United States, provoking a bitter debate about how American police forces treat non-white citizens that has drawn in President Barack Obama and his attorney-general, Eric Holder.

De Blasio on Monday called for a pause in protests until after the funerals of the two police officers.

Saturday's attack has left police forces around the Untied States on edge.

Police in Chicopee, Massachusetts, on Tuesday said they would seek to bring criminal charges against a man for posting the words "put wings on pigs" on his Facebook page, a phrase similar to the one used by the man who shot the officers in New York.

The gunman, Ismaaiyl Brinsley, who was black, in a social media posting linked his plans to the July chokehold death of Eric Garner, 43, in New York's Staten Island borough and the August shooting of Michael Brown, 18, in Ferguson, Missouri.

Brinsley shot himself to death after killing the officers.

De Blasio, who campaigned on a promise of repairing relations between police and members of minority communities, has been harshly criticised by the city's largest police union for being insufficiently supportive of police.

But the mayor angrily rebutted those claims on Monday, saying there was no conflict between opposing police violence and supporting police officers.

Emerald Garner, a daughter of Eric Garner, left a wreath at an impromptu road-side memorial swelling with flowers and candles that marks the spot where Ramos and Liu were killed.

"My dad wasn't a violent man, so to use his name to do something that's violent is definitely not something that my father would want," she told reporters.

Liu had married only two months before his death.

His widow, Pei Xia Chen, emerged from her Brooklyn home with relatives on Monday evening.

"This is a difficult time for both of our families," she said.

"But we will stand together and get through this together."