"They're murderers, criminals, and they are going to rot in hell where they belong," one man shouted.
About 1,000 police officers were deployed around the court in the Queens area after the verdicts were handed down.
Mike Oliver, Marc Cooper and Gescard Isnora fired more than 50 bullets during the incident, in which two of Sean Bell's friends were also wounded.
|Bell was killed inside his car outside a|
The three waived their right to a jury trial and decided to have the judge decide guilt or innocence.
Defence lawyers had said jurors in the borough of Queens were likely to be biased against the policemen due to the intense media coverage the case has generated.
Judge Arthur Cooperman said the charges could not be proved beyond a reasonable doubt.
"Questions of carelessness and incompetence must be left to other forums," Cooperman said.
Al Jazeera's Kristen Saloomey in New York said there was likely to be an appeal against the verdict and a federal investigation.
Saloomey said the officers still could be forced to pay compensation and may face disciplinary action.
Reverend Lennox Yearwood Jr, a community activist and president of the Hip Hop Caucus, told Al Jazeera that the verdict "shows that the rule of law can be broken and if the rule of law can be broken there is no law at all".
"In too many cases young people of colour who are gunned down in this country are in a position where they are literally looked on as animals in the street and the department of justice must speak out against these cases," he says.
"This mindset and disregard for life must stop."
'Armed and dangerous'
The eight-week trial centred on whether the detectives had reason to believe they faced imminent danger when the shots were fired and whether they made it clear to Bell and the two survivors that they were police officers.
On the night of the shooting, Isnora, the undercover officer who fired first, followed Bell and his two friends to Bell's car believing they had gone to fetch a gun to settle a dispute at the club.
The defence argued that the victims were drunken thugs who the officers believed were armed and dangerous.
Prosecutors had attempted to convince the judge that the victims had been minding their own business, and that the officers were inept and trigger-happy.
The case has revived painful memories of other police-related shootings in the city, such as the 1999 shooting of Amadou Diallo - an African immigrant who was hit by 41 bullets by police officers who said they mistook his wallet for a gun.