Police shot dead a man armed with a knife in a terrorist attack on London Bridge. The man, identified by the Metropolitan police as 28-year-old Usman Khan, fatally stabbed one man and one woman in an attack that began at a justice conference in Fishmongers’ Hall about 2pm on Friday. Two other women and one man were injured and taken to hospital.

The Guardian understands Khan was attending the Learning Together conference, organised by the University of Cambridge’s Institute of Criminology, and began the attack while attendees were returning for the afternoon session. Footage showed members of the public grappling with a man on the north bank of the bridge before police intervened and shots were fired.

Thomas Gray, 24, told the PA news agency he helped five or six other people who had wrestled the man to the ground. “I was brought up on rugby and the rule is ‘one in, all in’. I did what any Londoner would do and tried to put a stop to it.” Gray said the man was carrying two knives that appeared to be “taped to his hands.” He said he stomped on the man’s left wrist while another man kicked the knife away.

Other bystanders described seeing armed police officers surround the man, appear to look at his jacket, and then back away. Another video showed a body lying on the pavement with armed officers pointing their weapons from several metres away. The man was shot by specialist armed officers.

The Metropolitan police commissioner Cressida Dick said the attack started at Fishmongers’ Hall in the City of London, just by the northern end of London Bridge. Police were first called at 1.58pm and City of London police officers had confronted the suspect by 2.03pm. The police confirmed the suspect died at the scene and was wearing what upon later examination was believed to be a hoax suicide vest.

Assistant commissioner Neil Basu said police were searching the Staffordshire residence where Khan had been living since his release from prison. “This individual was known to authorities, having been convicted in 2012 for terrorism offences. He was released from prison in December 2018 on licence and clearly, a key line of enquiry now is to establish how he came to carry out this attack.”

Khan served eight years for terrorism offences connected to a plot to blow up the London stock exchange. He was subject to monitoring conditions after his release and wearing an electronic tag. At his sentencing in 2012 the judge said Khan could pose an ongoing threat to public safety.

In 2017 eight people were killed and 48 injured when terrorists used a vehicle and knives to attack people on London Bridge and in nearby Borough Market.