A man took a 1.5-metre narwhal tusk - a horn protruding from the whale - off the wall at Fishmongers' Hall to confront the London Bridge stabbing suspect, an eyewitness has claimed.
On Friday night, film director Amy Coop wrote on Twitter that she saw a man take the tusk off the wall at the historic listed building in the centre of London.
A 28-year-old man is suspected of killing two people and injuring three others in an incident that began at Fishmongers' Hall on Friday afternoon, police said.
The suspected attacker had been released on license after serving jail time for a terror conviction connected to a foiled plot to bomb the London Stock Exchange.
Video and photo posted to social media showed people struggling with the suspect who was later shot by police.
One man holds what looks like a large white stick, which Coop said on Twitter was the tusk. Another held what appeared to be a fire extinguisher.
Another social media user posted a photo of the Narwhal tusks inside Fishmongers' Hall. Euronews has not yet confirmed that the photo was taken in the historic building.
The Times reported that the man who grabbed the unconventional weapon was a Polish chef who works at the hall. The words Polish and Narwhal were trending on Twitter in the UK on Saturday morning.
Many social media users were pointed out that in the 2017 attack at Borough Market and London Bridge, a Romanian baker ran towards an attacker and threw crates at him.
Fishmongers had been hosting a prison education alumni event put together by the University of Cambridge's Institute for Criminology initiative "Learning Together".
Police said the suspect attended the event.
"I am devastated to learn that today's hateful attack on London Bridge may have been targeted at staff, students and alumni attending an event organised by the University of Cambridge's Institute of Criminology. We are in touch with the Metropolitan Police and awaiting further details of the victims," said Professor Stephen J Toope, the vice-chancellor of the University.
Police have called for people who were present at Fishmongers' Hall to contact them about what happened.
British author and historian Guy Walters posted a thread of information about narwhal tusks, stating that it is not a very cheap weapon and can be valued at tens of thousands of pounds.
Other social media users hailed those who confronted the suspect as "heroes".