It's the second terror attack in England in as many weeks, after the Manchester attack at an Ariana Grande concert on 22 May.
At roughly 10.30pm local time, a van was driven into pedestrians on London Bridge, before three men inside the vehicle alighted and started stabbing civilians at Borough Markets.
"Then after [London] Bridge, the people that were actually on the floor, I think after the van had crashed, I think that's when he must have been stabbing," a witness named Brian said, reports abc.net.au.
"I think the injuries were more from stabbing. I just saw a lot of blood. I remember just seeing a lot of blood. There was a lot of puddles. I think it was more knife attack than anything."
There have been reports of the assailants carrying large knives, with a witness saying he saw a blade of approximately 10 inches.
Another witness recounted how bottles, stools and chairs were thrown at the attackers to try to frighten them away.
"When I first saw them they went, 'This is for Allah', and stabbed this girl I don't know how many times — 10 times maybe 15 times — and she was going, 'Help, help me", and I could not do nothing," a witness said.
People inside restaurants at Borough Markets were placed in lockdown, with police later returning to evacuate.