Mrs May described the incident, which began with reports of a van hitting a number of people on London Bridge, as a "terrible incident".
One witness told the BBC she saw a speeding white van veering into pedestrians at London Bridge. They also issued safety instructions to those in the areas of the incidents, warning people to run, hide, and contact police when it is safe to do so.
The BBC showed a photograph of two possible London attackers shot by police, one of whom had canisters strapped to his body.
Multiple witnesses reported a vehicle veering off the road and hitting several pedestrians along London Bridge; others also said they saw a man with a knife.
Officers are dealing with reports of stabbings in Borough Market, where armed officers attended and shots were fired.
"Officers have then responded to reports of stabbings in Borough Market", police said.
Police declared the events at two locations - London Bridge and Borough Market - were terrorist related.
News reports that London Bridge has been closed as armed police gather.
He also tweeted, "Whatever the United States can do to help out in London and the U. We were eventually led out of the bar and some kind people working in a hotel let us come in to use the bathroom and to give us water".
Macron holds 'extremely frank' talks with Putin
Ahead of the visit, Macron told a French weekly that he was not "bothered" by leaders who "think in terms of power dynamics". But he also defended his March meeting with Macron's rival in the presidential race, far-right leader Marine Le Pen .
The Times of London is reporting tonight that one man has been arrested.
Witnesses also said they saw two men stabbing people outside the well-known Roast restaurant in Borough market according to The Guardian.
"I saw that man with a knife in his hand and after that a man started screaming so I knew something wrong was happening", he said.
He told Sky News: "It was about 10.15pm I was in the back of an Uber cab driving over London Bridge.there was somebody down on the pavement with a small crowd around them clearly very concerned".
The incident comes less than two weeks after 22 people were killed in a bomb blast following a Manchester concert. Police have closed a nearby railway station and told people to get away from the area.
MARTIN: And, Frank, we only have a little bit less than a minute left, but this is all coming just a month after three people were killed near Parliament there in London when a vehicle ran over victims.
Witnesses reported seeing injured people on the ground on the bridge. The attacker was gunned down by a police officer.
Grande and others are slated to return to Manchester on Sunday for a benefit concert.
"They're trying to get people out of the city now but when these things happen the first reaction is just the fear and confusion".