Dramatic scenes erupted at the Royal Courts of Justice in London as the decision by the jury of seven women and three men triggered outbursts from family and friends who branded police "murderers" and hurled abuse.
Large numbers of officers were on duty in the UK capital last night in a bid to avoid a repeat of the arson and looting during the immediate aftermath of the shooting.
Dozens of protesters gathered outside Tottenham police station chanting "racist murderers" and carrying placards which read "no justice, no peace".
Outside court, Mr Duggan's brother Marlon shouted after the jurors saying "f**k them", and angry supporters spilled from the courtroom and kicked at a door.
Mark Duggan, whose death sparked riots and looting across England, was gunned down when police stopped the taxi in which he was travelling in Tottenham, London, in August 2011.
At the culmination of the four-month inquest, the jury found that although the 29-year-old had a gun in the minicab, he most likely threw it onto a verge as soon as the car came to a stop.
Outside the court, Mr Duggan's family claimed he was "executed" and branded the judgment "perverse".
Mr Duggan's aunt Carole Duggan said: "The majority of the people in this country know Mark was executed. We are going to fight until we have no breath left for Mark and his children."
His brother Shaun Hall said: "We came for justice today, we don't feel we are leaving with justice."
Police watchdog the Independent Police Complaints Commission said it was looking at new evidence that had emerged from the inquest, and the Duggan family are now considering whether to try to get the inquest conclusion judicially reviewed.
Their solicitor Marcia Willis-Stewart said: "On August 4, 2011 an unarmed man was shot down in Tottenham. Today we have had what we can only call a perverse judgment.
"The jury found that he had no gun in his hand and yet he was gunned down.
"For us, that's an unlawful killing."
As her words were interspersed with shouting from a gathered crowd, she said the family were in a state of shock, adding: "They can't believe that this has been the outcome. No gun in his hand and yet he was killed - murdered as they have said."
Emotions boiled over outside the court building as assistant commissioner Mark Rowley made a statement. He could not be heard and protesters jostled journalists. The angry crowd shouted "murderer" and "murdering scum". Others called out "who killed Mark Duggan? The police killed Mark Duggan."
Mr Rowley said criminals had shot dead more than 50 people in the UK capital in the past three-and-a-half years. "No officer sets out to run an operation that results in someone dying. But the task our officers face in making split-second decisions when confronting armed criminals means there is a risk - a very small risk - that this will happen."
Mayor Boris Johnson said Met officers face incredible pressure, but have opened fire only six times in the past four years.