The attacker was shot dead by armed police.
Several people were stabbed by the knifeman on London Bridge before he was grappled to the ground and disarmed by members of the public.
It is believed two people were killed following the attack.
Footage on social media appeared to show one man being pulled away by armed officers before the suspect is shot at point-blank range.
Neil Basu, the head of UK counter-terrorism policing, said: "At approximately 2pm today, police were called to a stabbing at a premises near London Bridge.
"Emergency services attended including officers from the City of London Police and Metropolitan Police.
"A male suspect was shot by specialist armed officers from City of London police and I can confirm that this suspect died at the scene."
Mr Basu said he would give updates to the families of those injured "as soon as we can get that information".
"A number of other people received injuries during this incident. As soon as we can provide further updates on their condition, we will," he told the press conference.
"Our heartfelt sympathies go out to everybody who has been involved in this and is anxiously waiting for information on their loved ones.
"As soon as we can get that information, we will get it to you."