Assistant Commissioner Mark Rowley said a member of the public also received a gunshot wound in the hail of bullets from the eight officers.
Seven people were killed and 48 injured in the attack on Saturday, with 21 fighting for their lives in hospital.
Twelve people have been arrested in connection with the atrocity after an armed raid in Barking, east London.
Mr Rowley said: “Eight police firearms officers discharged their weapons.
“Whilst this will be subject to an investigation by the IPCC, our initial assessment is in the region of 50 rounds - in the region of 50 bullets - were fired by those eight officers.
“The three attackers were shot dead.
“The situation these officers were confronted with was critical - a matter of life and death - three armed men wearing what appeared to be suicide belts.”
Mr Rowley added: “As the officers confronted a terrorist, a member of the public also suffered a gunshot wound.
“Although the injuries are not critical in nature, they are in hospital receiving medical attention.”