Metropolitan Police assistant commissioner Mark Rowley confirmed that the person was shot during the police response and is being treated in hospital.
He also stated that many of the victims are foreign nationals.
Police are currently trying to trace the next of kin of all those killed and injured.
Mr Rowley also said an 'unprecedented' amount of ammunition - around 50 bullets - had been fired at the three suspects during the attack. All were killed by officers.
Although an IPCC investigation into the circumstances will take place, he described the situation as 'critical' and added that the attackers were initially feared to be suicide bombers.
Twelve arrests have now been made after police raids in Barking, east London.