A white van has reportedly hit no less than 15-20 pedestrians along London Bridge Saturday night in a suspected terror attack leading to seven people feared dead and 20 hurt in a 10pm local attack.
May said that "following updates from police and security officials, I can confirm that the awful incident in London is being treated as a potential act of terrorism". Witnesses also reported seeing people being stabbed by at least one man.
BBC reporter Holly Jones, who was on London bridge, said the van was driven by a man and was "probably travelling at about 50 miles an hour".
Witnesses claim a white van veered off the road and hit five or six people at London Bridge.
Mayor Khan said the incident was still unfolding.
Witnesses reported seeing injured people on the ground on the bridge. "As I was thinking ... which direction should I run, the van swerved across the other side of the bridge from me".
British transport police say there are multiple casualties.
UKpolice rushed to an incident on London Bridge on Saturday after witnesses said a van ploughed into pedestrians and another witness told Reuters she saw people with throats cut on London Bridge.
In March, a knife-wielding man went on a deadly rampage in the heart of Britain's seat of power, plowing a vehicle into pedestrians on London's Westminster Bridge before stabbing a police officer to death inside the gates of Parliament.
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And in the popular food market in south London, witnesses said they saw a man with a large blade.
Police have advised people to keep away from the area.
Ms May was chairing a meeting of the Government's emergency response committee COBRA, Downing Street said.
A woman, who did not want to be named, added: 'We were in the restaurant and we just saw three guys come in, stabbed someone in the face and someone in the stomach.
Terrifying footage from the scene shows police officers shouting "Clear the scene now!" as panicked pedestrians flee.
A police helicopter is at the scene along with numerous ambulances and police vehicles.
Armed officers responded to the reports of the stabbings and confirmed "shots have been fired", Metropolitan Police said.
Britain's official terror threat had recently been lowered from "critical" after a bombing killed 22 people at an Ariana Grande concert in Manchester.