Witnesses urged them to take action against knife-wielding Michael Adebolajo, and Michael Adebowale, who was armed with a gun, in the wake of the murder.
In the 14 minutes from the first 999 call that it took for armed officers to arrive, several members of the public had come close to Adebolajo, who stood ranting at cameras with bloodied hands, and his fellow killer Adebowale.
Head of counter-terrorism for Scotland Yard, assistant commissioner Cressida Dick, said that the unarmed officers had faced an agonising decision.
"It is a horrible dilemma for any police officer in such a circumstance whether to intervene or not intervene," she said. "I think there was good command and control, there was good decision-making, and my personal view is that it is extremely likely that if an unarmed officer had attended and tried to intervene we would be dealing with more murders."
When armed officers arrived on the scene, they had only seconds to react as Adebolajo charged at them clutching a meat cleaver, with Adebowale looking on and brandishing a gun.
The female driver of the BMW was unable to reach her Glock pistol in time, leaving her colleagues to fire at Adebolajo, who was sent flying into the air by the force of the bullets, and then shoot Adebowale.