The gunman who shot dead two New York City police officers told passers-by to "watch what I'm going to do", moments before the attack.

Chief of Detectives Robert Boyce said witnesses recall Ismaaiyl Brinsley asking them to follow him on Instagram, before making the chilling warning.

Instagram posts attributed to Brinsley, 28, have been described as "very anti-police".

One appears to threaten to carry out an attack in retaliation for the death of Eric Garner, who was killed in a New York officer's chokehold, sparking weeks of mass protests.

Part of the caption read: "I'm Putting Wings On Pigs Today. They Take 1 Of Ours ... Let's Take 2 of Theirs."

Eric Garner's family have condemned the killings, denouncing revenge attacks.

Joined by civil rights activist Reverend Al Sharpton, Mr Garner's mother Gwen Carr told reporters: "I’m standing here in sorrow about losing those two police officers, that was definitely not our agenda.

"We want you to not use Eric Garner’s name for violence, because we are not about that."

A candlelight vigil was held for the two dead police officers on Sunday, a day after they were ambushed in their squad car while on patrol in the Bedford-Stuyvesant neighbourhood of Brooklyn.

They have been named as 32-year-old Liu Wenjin and 40-year-old Raphael Ramos.

Police have said Brinsley shot the pair in the head with "no warning, no provocation".

"They were quite simply assassinated," Police Commissioner William Bratton said.

Brinsley then ran inside a subway station and fatally shot himself in the head as responding officers pursued him.

Details have now emerged surrounding his long criminal record and apparent history of mental instability. Brinsley shot and seriously injured his ex-girlfriend in Baltimore, Maryland, shortly before travelling New York to carry out Saturday's attack.

He had at least 19 arrests in Georgia and Ohio and had spent two years in prison for gun possession.

Police said Brinsley's mother believed he had been suffering from an undiagnosed mental illness. He also reportedly tried to kill himself last year.

The killings have sparked fears of further revenge attacks amid heightened nationwide tensions over the deaths of Mr Garner and another black male at the hands of white police officers.

The unarmed teen Michael Brown was shot dead in Ferguson Missouri back in August.

Decisions by US grand juries to not charge either of the officers involved triggered widespread protests.

New York police officers have been told to take extra backup when responding to radio calls and have been urged to wear bulletproof vests when out on the street.

On Sunday, a police officer in Florida was shot and killed in what is believed to have been an unrelated incident.