The three attackers drove a hired van into pedestrians on London Bridge on Saturday night then ran into the bustling Borough Market area where they stabbed numerous people. Seven died and 48 were taken to hospital.
The attackers were wearing what were later found to be fake explosive vests.
"Eight police firearms officers discharged their weapons ... 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," Mark Rowley said.
Most police officers in Britain are not armed and it is unusual for officers to fire their guns.
"The situation these officers were confronted with was critical: a matter of life and death," Rowley said.
"Three armed men wearing what appeared to be suicide belts. They had already attacked and killed members of the public and had to be stopped immediately.
"Indeed I am not surprised that when faced with what they must have feared were three suicide bombers, the firearms officers fired an unprecedented number of rounds to be completely confident that they had neutralised the threat that those men posed."
Rowley said police were making significant progress in identifying the three attackers, but gave no details. He said police were increasingly confident the attack had been carried out by three individuals, but needed to establish whether others were involved in the planning.
He also said security arrangements for forthcoming events were being reviewed and the public could expect to see additional police, both armed and unarmed, across the capital.
"You will also see increased physical measures in order to keep the public safe on London's bridges," Rowley said.
Saturday's attack was the second to involve a vehicle driving into pedestrians on a bridge in London in just three months.
In March, five people died after an attacker drove a car at pedestrians on Westminster Bridge and stabbed a policeman in the grounds of parliament. The attacker was shot dead by police.