Three terrorists left the van and attacked onlookers with knives.
May will chair a meeting of the government emergency response committee on Sunday, Downing Street officials say.
A white van mowed down pedestrians on London Bridge shortly after 10pm before the attackers, wielding knives and a machete and wearing fake suicide vests, stabbed revellers enjoying nights out in nearby Borough Market. Three male suspects were shot dead by police, and 12 people have been arrested in connection with the incident.
"They went 'This is for Allah, ' and they had a woman on the floor". She called for worldwide agreements to regulate cyberspace and said Britain needs to become more robust at preventing the internet from being used to the advantage of extremist groups. Police tweeted a warning telling people in the area to run to safety, hide and then call police if it is safe to do so.
The attack lasted about eight minutes, police said, but seven people were killed and at least 48 were hospitalized.
He said: "If it is, I am completely shocked". French and Spanish citizens were among the wounded.
Canadian Prime Minister Justin Trudeau said a Canadian was among the dead. I've said hello in passing more than 50, 60 occasions. Raids are going on elsewhere in the city. "Stand up and show us your hands!"
Grande and other acts were due to give a benefit concert at Manchester's Old Trafford cricket ground on Sunday evening to raise funds for victims of the concert bombing and their families.
ISIS has not officially claimed responsibility for the attack, but the terror group's weekly Al Naba mag contained an article praising last month's attack on the Manchester Arena.More news: Grande returns to Manchester for benefit concert
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"We saw injured people on the road, injured people on the pavement", he told Sky News.
Theresa May earlier today vowed to defeat our enemies, and said "enough was enough" with Islamist extremists. After that attack, Britain's official terrorism threat level was raised from "severe" to "critical", meaning an attack may be imminent.
Police urged the public to "remain calm but vigilant" as the investigation continued, and asked for the public's cooperation with the probe. Police have responded to reports of a van hitting pedestrians on London Bridge in central London.
As emergency services sped toward London Bridge, reports came in of multiple stabbings nearby, in the area around Borough Market, a popular gourmet food market surrounded by fashionable bars and restaurants.
Renan Marquese, a sous-chef at a tapas restaurant, said he was working when he heard chaotic sounds outside the restaurant.
At 4 a.m., police said six people were killed. Police escorted them out 30 minutes later.
"It was really scary", he said.
The Met Police are treating the incident as a terrorist attack.
At least one officer received injuries that are not life-threatening, police said.
"There were three of them. the ambulance came and took them away". He said he saw someone jump into the water in fear. "And it turns out to be a heroic guy who saw what was happening and just bombarded these bad cowardly people with stuff".
"Perpetrators are inspired to attack not only on the basis of carefully constructed plots. and not even as lone attackers radicalized online, but by copying one another and often using the crudest of means of attack".
"One man tried to flee and they shot him outside my house".