Witnesses have described the moments a van left the road in central London and ploughed into pedestrians while men calmly went about stabbing people.
About 10.40pm UK time (9.40am NZ time), reports filtered through of a van leaving the road at London Bridge and hitting pedestrians.
Soon after, police confirmed they were also attending incidents at nearby Borough Market.
At London Bridge witnesses described a terrifying scene.
BBC reporter Holly Jones was there at the time of the attack and said she saw the van veer into pedestrians and that she could see six casualties.
The BBC reported other witnesses saw men jump out of the van and begin stabbing people.
Photographer Gabriele Sciotto was there when the attackers came into the street from London Bridge and took several photographs of the attackers after they were shot by the police.
"They were almost coming towards me, I tried to run away and then they were going towards the main street and at that point more police came over and I guess in around two or five seconds they shot all the three men down."
Lara Al-Ostta told The Guardian she was having drinks with a friend in a bar underneath London Bridge when people came running into the pub and telling everyone to run away because "people are stabbing each other".
At nearby Borough Market husband and wife Ben and Natalie told BBC Radio 5 they saw a man being stabbed.
Ben said: "I saw a man in red with quite a large blade, I don't know the measurement I guess maybe 10 inches. He was stabbing a man ... he stabbed him about three times fairly calmly.
"It looked the man had maybe been trying to intervene but there wasn't much that he could do, he was being stabbed quite coldly and he slumped to the ground."
The attacker and another man walked off quite "boldly," he said.
"A table was thrown, a bottle was thrown at the individual with the knife and then we heard three gunshots and we ran," he added.
A security guard who oversees pubs in the Borough Market area says he saw four people stabbed by three attackers.
The man, who was deeply shocked and asked not to be named, told the BBC he was at the Market Porter pub when a colleague at another bar said there was a stabbing at the Borough Bistro pub nearby.
He said as he went towards it "everybody started running" and there was screaming.
The man said there were three attackers, one of whom had a long knife and was stabbing people, including a young woman.
The BBC's Tom Simons talked to a bar security guard who worked in one of the bars that was attacked and after getting a call from a colleague at another bar went to see what the commotion was about, thinking it was likely a fight.
"And says he saw three men one with a red and black top, he said this man has a long knife and was attacking people.
He said he saw one woman in her early twenties be stabbed and go to the ground.
"He said that he and his colleagues tried to throw chairs at these men to try to stop them. He was deeply shocked when he spoke to me, I don't doubt he saw something very distressing."
Will Orton, 25, who was at another pub nearby, a lot of people came running inside.
"We didn't really know what was going on.
"We thought maybe there was a fight or something outside. And then there were almost hundreds of people coming inside.
"The bouncers did a really good job, they shut the doors and locked everyone in," he added.
"There was panic - it seemed like it was literally outside the door.
"People were coming inside and saying they had witnessed people being stabbed."
BBC producer Emily Craig was also in the nearby Borough Market area, sitting in the basement of a restaurant.
"Staff in the ground floor started running down the stairs. They told us that they had been instructed to shut the doors, and there was a kind of wave of panic from the people in the restaurant.
"Then people from the ground floor were moved downstairs to the basement, and then we heard gunfire outside and some people saw armed police patrolling the area around the restaurant."