The white van then continued to drive to Borough Market, and three men left the vehicle and began to stab people, including a British Transport Police officer.
A speeding van first mowed down several pedestrians on London Bridge before three men jumped out of the vehicle and started stabbing people with "long knives".
That witness said the van hit five to six people.
Three men have been shot dead after killing six and injuring 48 more in a van and knife attack in London.
One witness told the BBC she saw a speeding white van veering into pedestrians at London Bridge.
An eyewitness told Sky News he saw people who seemed to have been run over and people being placed in an ambulance covered in blankets.
Spectator journalist Will Heaven was at London Bridge shortly after the incident and wrote on Twitter that he had seen casualties - one on the pavement and one on the edge of the road.
Assistant Commissioner Mark Rowley said: Armed officers responded very quickly and bravely, confronting three male suspects who were shot and killed in Borough Market.More news: What is Pentecost Sunday and how is it celebrated?
Train travel within the city has been severely affected by the attacks, with London Bridge station and Borough Tube station remaining closed until June 4 (local time).
The violence turned a summery Saturday night in an area full of bars and restaurants into a scene of panic and chaos, with officers running through crowded streets screaming for people to flee and lifeboats drafted to help clear the area.
"I was walking out of the London Bridge station and there were police", Caroline Breniere said.
As dawn broke over the capital, a large area on the south bank of the River Thames remained cordoned off. Police told people to avoid the area, leaving tourists and revelers struggling to get home.
"As with the rest of the world, the Australian team is shocked and saddened at the events that have occurred in London overnight and our thoughts are with those directly involved and their loved ones", CA said.
At the same time, it was also hard for them to check into another place, as the hotels were warned by police to not take in new guests in order to not compromise the security of those already there, Boyko said.
He stabbed a police officer to death before being shot dead by a ministerial bodyguard.
"Today we are united across borders in horror and mourning, but also in determination", Merkel said in a statement.More news: India committed to climate change pact: Harsh Vardhan
It was the third major attack in Britain in the past three months.
"To our neighbours in the United Kingdom I send the sympathy and support of the Irish people, and in particular the heartfelt sympathy to the families of those who have lost their lives, and our thoughts and wishes for such recovery as is possible to the injured and their families".
Metropoliton Police have since confirmed the two incidents are being treated as acts of terrorism. Grande and other stars are scheduled to perform a benefit concert for victims Sunday night.
Vowls also saw people pelting the attackers, and said he joined in himself.
"I saw two young ladies in their 20s who both died in front of me".
"But the last thing we need is people overreacting or beginning to take out their frustrations on other people in other communities or in their own communities. It looks potentially they could have been thrown over".
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